I don't think the next iPhone will cost more than $199 w/ a two-year contract.
(Unless you get an iPhone with more storage space, of course.)
The iPhone will never again cost $499, like it did when it first came out. The reason why it originally cost so much is because it didn't come with a contract. After the current version was released, AT&T subsidized the cost of the phone, meaning that the initial price was reduced but required a two-year contract in return.