I got my motherboard off ebay for $400, but it also came with palmrest, bluetooth, and bottom base, and it was for the XPS M1710. The M1710 and the M90 use the same motherboard. The problem with that is they're very rare finds; i think the one I got was the 2nd type to be sold on ebay. The first m1710/m90 board/base was like $800 without videocard, memory, processor, etc.
Another option would be getting a 9400 base/motherboard off of ebay. They go for like $200-230 and the only difference between that and the m1710/m90 boards are the BIOS setup and the d/dock expansion port. But if there is one on ebay, I say go for it.
edit: I just read that you had no luck with ebay. Try ebaystores (www.ebaystores.com
) instead. I got mine off this guy. His ID: dnksalescomputers. Try him, good luck.
but the ones listed don't have the d/dock port from what i could see in the pictures