Hey,
The demo is 120 images + images which is around 20% of the whole game. It as multiple routes you can go down to give you an idea what the whole game is like without actually giving you the whole game.
My normal price point for images is 10 images for roughly $1. Though I often go over this with my comics for the sake of story. Measuring Up has 600 images in it which is insanely good value. I think I released the game 5 years ago now and I've kept the price steady as I still think it's good value for money and I'm still told people really enjoy it.
Out of 104 ratings on Gumroad 85% gave it 5 stars:
https://ohh.gumroad.com/l/mu600
I get it though. $20 ain't cheap for most people and I won't begrudge anyone who thinks it's not worth it. I'd encourage you to retry the demo whether you intend to buy or not, just to at least enjoy all 120 images. That's a lot of content for nothing.