the more you pay the better girls you will get usually. A super hot college girl would rather make $100 waitressing for 4 hours at a nice bar than $100 to have sex on camera and have her pictures all over the 'net. Whereas a not so great girl might think that is a damn good deal.
That being said it depends on the girl. I have one late 20's lady who is really nice girl next door material and has always wanted to model for an adult site so she was excited when she saw my posting, she does cam and stuff and for her it is just really exhibitionistic so she is more than happy for $250-300 for a full day shoot (about 8 hours including breaks, lunch, etc).
Something that might help a girl decide is to invest in a few really high quality wigs (ones that don't look like wigs) b/c it might make her feel more "safe" as she feels she is hiding her true identity. I recommend
http://www.wigsalon.com for high quality wigs that are really cheap. Steer away from blonde though because blonde wigs tend to look fake because they are too shiny. And remember to pick up at least one wig designed for smaller heads otherwise if you have a petite girl the wig will look really fake and you might not want to use the photos.