At least for the character drawings, instead of saying 10-15 dollars I would give a set amount. Like $10 regardless of the character design difficulty. Maybe only an extra 5 for more characters added, but 8 is reasonable. I think giving set amounts will help, because then customers are more sure by what they see and they aren't warded off by higher numbers.
You should really bump the prices up higher. Even the the character are to cheap. I've seen people pay at least $50 or more for that quality. Also, you should do by how complex the character is as well. As for ref's.Ever thought of doing the full-view ref sheet? The one with at least 4 different angles consisting of back,side,front and a 3/4 view? Lots of companies and productions really like to see that kind of ref's. You can probably charge 80-100$ for one of those. I honestly think artist on here are paid to little. I commissioned she was asking 15$ for almost full body consisting of 4 characters ,colored traditional as well as sending the original. I told her that was to cheap. I gave her 50$ and I still think that was not enough for her quality of work. Look at that beautiful art! She should be doing comic or games. Believe me, I have seen people pay double for very shitty art that does not even come close to you quality. Really make your prices higher.
[Character Drawings] $30 and up depending upon complexity / number of characters [Creature Designs] $40 and up depending upon complexity [Reference Sheets] $50 at the lowest. Those things take freaking forever and yours are quite pretty!
Stop thinking your art isn't worth it! Your stuff is pretty high quality and I'm honestly surprised at how little you are paying yourself for these commissions!
You should record how many hours it takes you to make your art, then decide how much you want to make per hour. Don't be skimpy with what you pay yourself. You obviously have a lot of skills and talent. Your work is beautiful and the drawings have so much detail in them. Don't undervalue your work!