I just checked the web site and after the free ones, it is 15 cents for 3x5 and 4x6, 5x7 is 89 cents and 8x10 is 2.89. They also do poster size enlargements and photo packages.
And BTW you have to purchase the 100 free within 14 days of joining...here is the "official" disclaimer...
100 FREE prints offer:
Upon joining dotPhoto, you will receive 100 FREE 4x6 prints to be used within 14 days. Shipping not included. Limit one per person, e-mail address, household, family, camera, credit card and computer. All credits must be redeemed in your own account and will expire within 14 days of joining. To check the status of your print credit, click on "My Account" after you join.