Out of the loop - MTB components

[laura*]

Shimano 105 components for a road bike. Is there a similar equivalent for mountain bikes?

The corresponding MTB group would be either Deore or SLX (nee Deore LX). The next group down, Alivio, has been cheapened.

What happened to XT and XTR?

They're still around. XTR is perhaps for the sponsored racer who wants the ultimate race component, but doesn't care about longevity.

What should I expect to spend for a decent mountain bike?

Probably significantly into the four digit range.