$25 - $100 seems a bit much for digital paintings that you aren't even going to print out. it's understandable that you need money for college, but i think you might get more interest if you dropped your prices a bit. you're asking just a bit too much.
Despite the fact that you spend 5-10 hours on them, if you browse around dA, digital painting that are very similar to yours are less expensive. Plus, you're doing a paint-over, not starting from scratch.
Drop your price mate, I spend over 20 hours on a pastel drawing of just a head and charge AU$100, not much at all, but I get ALOT of business charging less than anyone else so I make up the money through having more clientel, then I can up my prices little by little once I've got clients and word of mouth established.
Start out low, even lower than what you think they're worth, then build up as you get more interest