First, it will be significantly more expensive to buy the phone than at Cricket. Cricket's iPhones run $399.99 for the 8GB iPhone 4, and $499.99 for the 16GB iPhone 4S. Virgin Mobile will charge $549 and $649 for the same devices. Guess what? That's the same price as for an unlocked iPhone (sans contract, in other words), meaning Sprint's not giving you a break like Cricket is.
In terms of the service plan, though, you do get a break. Cricket requires iPhone users to sign up for a $55 per month unlimited (sort of) talk / text / data plan. It's only "sort of" unlimited because the data, as is typical of many carriers nowadays, is throttled after 2.3GB.
Customers can also use their iPhone as a mobile hotspot through Virgin Mobile for an additional $15 per month. For Virgin Mobile, unlike its parent company, unlimited data doesn't mean unlimited. It means 2.5GB of data before throttling.
Virgin Mobile's iPhone goes on sale on June 29. More information can be found at its website.