Amanda, I gave Bob a suggestion for his specific situation, and I can tell you what I do, but I am the last person who should give pricing advice generally. You would be better off reading the DPW help articles on pricing, some of these sites, looking at the prices of comparable work by others, and deciding what's right for you. There are several factors, and the answer is different for everyone.
I try generally to price by size, though I tend to lower my price per sq. in. as the size increases. (To me, at roughly $2 per square inch, an 8x8" painting for $125 seems very reasonable, but $1,150 for a 24x24" painting seems more than I could expect to get--though I'm often told my prices are too low.) As David's articles suggest, internet pricing often is lower than gallery pricing--which is OK only if you're not offering similar work for higher prices in a gallery and undercutting your gallery.
Awhile back, I began setting my starting bid at about half (give or take) of what I ask for paintings offered in my DPW Gallery or at sales events. If all goes well, I may raise prices gradually. Hope this helps.