The only thing that causes Blessed Water to increase in cost is the day itself. Not sure what the exact formula is, but the cost of BW doubles at about day 26 (and it's slightly over quadrupled on day 80). It's entirely possible to be able to afford all 5 BW on Nightmare on Day 1 without NG+ bonus cash (admittedly you won't have much money left for anything else), so if you are going to buy them it's probably best to do it then, as it's never going to get any cheaper.
Shit. Maybe I ought to restart from the beginning... my income still isn't great, and I'm sitting at 11 corruption on Day 36, still unable to kill any of the Young Avian, the Goblin Shaman, or the demon on the road to the northern mine. My lewdness is only 4, and I've been able to sort of control my willpower, but I don't have a way to get the corruption down.
EDIT: I tried restarting from the beginning, the idea being that I would copy/paste the saves from my current run into a separate folder in case I needed them again. I left behind the save from when I got past the intro. ... Now the game no longer recognizes these save files. I tried making a save to test, and that worked, and then when I tried pasting in the files from before again, it now shows only that save slot, with the old save there. It turns out I can just skip the intro anyways, but I'm DEFINITELY going to need to make backup folders for my old saves (I had already used like 75% of the available slots just playing several hours yesterday), so what's the proper way to do this?
EDIT2: How are you affording all five blessed waters on day one? After turning in the Star Metal and collecting the herbs for the tavernkeeper, I only have 435 gold, which only gets me two blessed waters. ... Oh, I checked a walkthrough, apparently there's a different place I could sell the star metal for 1000g instead (presumably without the other benefits of selling it to the workshop). Well, shit, guess I have to start over again. No way the extra 20g per work session is gonna make up for that price difference. ... But this walkthrough is assuming one is playing on normal difficulty, who knows if this is even possible on nightmare... of course, it's also assuming 100% purity, which I'm not doing, I'm just trying to compromise as little as possible. (... Yeah, it wants me to defeat the duelist on *day 1*, that's obviously totally impossible. I do scratch damage to him and he kills me in three hits even while I'm guarding.)
Admittedly this involves avoiding a lot of annoying fights (read: save scumming), but major sources of possible Day 1 income includes:
(ED: Just ran through this myself)
50 G from the skeleton in the Abandoned Shrine, 150G from Hydrangea (Lack of Flavor) quest, 500G from the 2 Star Metal (one in the semi-hidden hole/cave in the south part of the central forest, one in northern forest cave), 100G from the chest in the lower northern forest, loose change around the Ogres and Mothercrow (~176 total), and 500 G from the chest next to the Ogre Commander.
In Normal you can also count the two Duelists, but as you say, in Nightmare it's kind of a wash since even goblins will wreck you at first (tbh even the goblin fight with Albrecht is a bit RNG-heavy). Still, that will put you at about 1300-1476 G (depending on if you collect all the accessible coins on the ground), which is sufficient to afford all 5 BW @ 1040. I don't feel like it's an absolute necessity to buy them, but if it's part of your plan it's best to do that Day 1.
I *can* fairly reliably do the goblin fight with Albrecht on day one if I'm at full health, but fighting goblins by myself is a death sentence unless I get both backstab advantage on them AND they miss several times (very unlikely). EDIT: actually I'm mistaken, I think I did that fight on day 2 (when Aura is level 2 instead of level 1). Hrrm. Doing it day 1 is a lot harder, especially since it appears that Albrecht is set to be whatever level Aura is... and if I can't get past this fight, I can't get to the northern forest on day one. :/ EDIT2: Okej, managed to pull it off somehow.
Huh, I didn't even realize there was a cave in the northern forest. I don't think I've gotten that far - it looked like I had to fight past goblins and ogres to get through there (I did manage to get through part of the way and find a chest with 100g in it, but I didn't see any crow-like monsters so I'm assuming the one you're talking about is something else). I'll look it over again.
It's probably not absolutely necessary to buy all the blessed waters immediately, but income after the initial burst is very limited and they only ever get more expensive, so I figure it's something to aim for. It's probably not strictly necessary in my run since I'm going for reducing lewdness as much as realistically possible rather than trying to do absolutely zero lewdness (probably totally impossible on nightmare difficulty), but they'll help, and I'm assuming that a few hundred gold isn't all that much once you get later on in the game.
How long is the entire game supposed to take (in terms of game days)? Is there any built-in time limit? (ie, it'd be logical for there to be a hard limit to how many days this can go on since presumably Alicia's plan is screwed if it takes so long that they all graduate and go to separate universities [unless Alicia can just buy her way into whatever super-prestigious school Aura nerds her way into])
EDIT: Also, would you recommend selling the star metals for 1000g, or giving them to the workshop for 250g? It'd take 38 sessions at the workshop to make up the difference of 750g, which is a lot, hence my question about the total length of the game. If I'm going to be here for hundreds of days, then the workshop is the clear long-term option, but if the adventure is over within one school term (as I'm assuming it would be since these characters must be seniors at their age) I'm not likely to do enough workshop jobs to make it worth it (unless the side benefits of workshop level make up for it).
EDIT2: I found the other star metal and gold you mentioned. Currently I'm sitting at 976g before selling the star metals. It appears that selling the star metal to the workshop takes the rest of the day, so I can't sell it to them on the same day I join the adventurer guild (I guess I just stand around awkwardly while they work instead of leaving them to it lol). 976g isn't quite enough to get all five holy waters, but it's close. Hrrm... I could sell off the Vitality Potion I found, I don't imagine I'll be using that since it hits willpower (then again, if that +5 max hp is permanent, perhaps that's worth it...)
Aside from guarding even more loot, the Ogre Commander has a 500 gold bounty and is one way to complete a quest that gives +10 experience(level 2 get?) and unlocks the refugee camp. It also drops a very strong(by early/mid game standards) DEF-boosting monster material for either Aura or her future slime partner. Using the collar to get an early kill on the Ogre commander can be viable as an alternative to taking a few lewd options for gold if you're trying to protect Aura's innocence(and your victory score). Especially if you know fire(since this lets you skip combat when harvesting the star metal in the refugee camp).
The collar will also reset Aura's hp/mp so you can get more done on your day 1(or day 2/3).