one of the biggest complaints about ebooks from customers is they are priced too high (often the same price as hardcovers, which is pretty ridiculous). it's refreshing for once to find a publisher who is pricing on the low side.
as to your question, i think your prices are very good, especially if your authors or your publishing house are relatively unknown. i believe that baen books usually prices their drm-free ebooks around 5 or 6$ and everyone seems to agree that is reasonable. yours are a bit lower than that, but in the same neighborhood. i think the advantage of having lower prices is that people are willing to take a chance on a book they are not sure they will like if it's not too expensive, so you could get a lot of impulse buys. so your best plan is to try to keep prices low to sell as many as possible (i know : this is totally unexpected, right ? never occurred to you at all before ?
).
a lot of people have said the biggest danger facing authors is obscurity. have you tried promotions like the first two chapters of a book (or even a whole book from time to time, like the first of a series) to download for free on your website, so people can try it and see if they like it ? you could also post about it here for instance in the "deals and promos" forum. you could even try putting the promo on feedbooks.com : be sure to mention explicitly in the book description if it's only the first two chapters, and make sure the website where people could get the whole book is mentioned in the description and again inside the ebook, at the beginning and at the end. both mobileread and feedbooks get a lot of traffic and you might get some new readers that way, and it's free. if you have a new sample every month (hyped in your newsletter) you could get a lot of repeat customers as well as new ones.
another way to spread the word is to include samples of other books from your catalogue at the end of all your books ; the first two chapters of the next book by that author, or a similar book that might appeal to the same readers. that's free too but could easily get more people interested in your books. maybe you do this already ; i don't read much of the genres you publish so i don't know your company, but i thought i'd mention it just in case.
anyway, nice to see a publisher who seems to understand how ebooks *should* be. i wish there were more like you. thanks for posting.