DotPhoto is something friends of mine have used. You get 30 prints free (with a catch) for signing up and then 3x5 or 4x6 prints are $0.29/print.
The catch is the 30 prints come 10 per month for two months and the last 10 if you sign up for a paid subscription program. The subscription program lowers the per-print prices. For instance, if you sign up for a $4.99/month subscription your prints go down to $0.19/print for 4x6. Of course, larger prints also get a discount.
Anyway, I saw prints from them and my friends were happy.
Another online store (I have no experience with them) is
http://www.snapfish.com/ - Looks like you get 10 free and then "as low as" $0.25/print. Maybe there's a catch there, I don't know.
Amazon seems to have partnered with "Ofoto" (
link here). Prices a little higher, $0.39-$0.49 for 4x6 prints depending on volume.
Many stores such as Target and Walmart are now doing cheap prints (~$0.30/print) in the store as well. Walmart seems to have an online photo development deal and also you can submit them online and then go to the store for pickup to avoid shipping fees.
Oh, and my recommendation is not to bother with a photo printer unless you need your prints immediately. You really will find it hard to beat a price of $0.30 per print in terms of cost for the print supplies, and you also have the initial investment in the printer. If you need prints immediately, though, I understand why you would need one.