The pricing of these cards can vary quite a bit based on rebates; specials; etc. With sales, rebates, etc. the relative pricing will likely vary frequently.
However, there's another possible factor here: The 9800GT comes in two "flavors" --> rebadged 8800GT's (using the same G92 GPU and the same 65nm fabrication); or the new 55nm fabs. You need to look at the tech specs for the specific make/model you're considering to know which of these it is.
==> But either way, the 9800GT is essentially just an 8800GT with a new number ... and will have VERY similar performance. So I'd just get whichever one is more economical -- in this case, the 9800GT.