What type of bike (year, make, model) are you trying to put this on? Are you talking about front or rear or both?
How many speeds are currently on your bike?
If you could answer those questions it could narrow down your selection.
A shimano 'deore' level derailler is trailworthy and would run you $25 for the rear and $10 for the front.
As a side note as I was looking up prices, a x.0 rear mech goes for $200?? DAMN!!