If you lose your phone and you're still under the original 2 year commitment, the unsubsidized prices will be $399/$499 depending on which model you buy, but you will be required to extend your contract for two years. There will also be an unsubsidized price coming in the future of $599/$699 where you don't have to sign a contract at all.
Check out paragraph three of yesterday's
press release from at&t:
Existing AT&T customers who are not currently eligible for an upgrade discount can purchase iPhone 3G for $399 for the 8GB model or $499 for the 16GB model. Both options require a new two-year service agreement. In the future, AT&T will offer a no-contract-required option for $599 (8GB) or $699 (16GB).
Hope this helps.