XT would be way overkill - what is the largest rear cog you are going to use?
A long cage Sora will work for most riding. $25
Shimano RD3500 Rear Derailleur No PCKG > Components > Drivetrain > Derailleurs | Jenson USA

If you need more capacity Deore is the best bang for the buck - $33
Shimano Deore M591 SGS Rear Derailleur > Components > Drivetrain > Derailleurs | Jenson USA