It must be so hard to price! I recon you just have to pick a price to start. If you don't get (m)any takers you can lower it and if you get lots you can think about increasing it. I assume that's how people come up with prices! $25 seems pretty reasonable! I guess you could also take into account how much time each one takes, and if you want to gather interest to begin with, you could offer some lower prices things to get started maybe. I have never bought art so I have no idea what things go for!