Somebody will probably (well, maybe) be along with a more ideal answer, but if you dig around ebay, there are a lot of 9-spd rear derailleurs still around. In fact, just the other day I scored a NIB/old-stock LX for under $25 delivered.
Also, I have a Trek 9spd with all Deore stuff on it, and I'm pretty happy with that level of shifter/RD (and I've owned bikes with XT/XTR). I'm also a fan of the old 9-spd stuff, and am annoyed with the apparent industry race to 20-speed.
Anyway, it may be lower-end than you're looking to go, but you can get the "newer" 9spd Deore rear der. from places like JensonUSA for $38. Reviews are great, with one guy saying he used one to replace his 14-year old XT, and he thought it shifts better than when the bike was new.
Just tossing that out there.