$99 is what - about £60? That could well be the price for OEM Home Edition. In the UK WinXP Home Edition OEM sells for about £55 (what I paid for it). A boxed, retail version is roughly double that.
Vista will probably be roughly the same I expect, but I wouldn't be surprised if it sells at a premium at first for the retail version (to rip-off early adopters who can't wait to be Beta-testers). They won't price it too high for OEM as Microsoft will want the main system builders to include Vista on their machines (it'll be embarassing for MS if they don't).
There will be many flavours of Vista, though, and I doubt if even Microsoft have the prices finalised yet.