I've been thinking about this economy thing for several days and I don't think it's necessary. Of course, go ahead and create one for use by those who want it, but here's what I plan to try:
Fundamentally, this game's mechanic is about trying to tick the Promise boxes faster than you tick the days. Money, when you have enough, simply helps solve a problem faster than creating what you need from scratch.
In a cozy story, you have enough money, but using it involves a skill. Shop/Barter Wisely, a skill that can be upgraded like any other. On a weak hit, you buy an incorrectly-sized doohickey and must return to the hardware store (time passes). On a fail, you get distracted and don't buy the intended item at all.