Yeah ^
If you take a look at Trek for instance the Frames are the same from low end up to the high end. The Trek 4300 I had the frame was identical to the 6700.
To me the most important thing when it comes to frames is whether or not the Geometry fits me. Gary Fisher makes a bike with a long top tube which is good for me because I got a long torso but shorter legs.
I just upgraded my old GF Marlin with a XT crank (barely used from eBay), LX RD, Deore FD and SRAM Attatck shifters for like $250. I spent about another $150 on a Headset, Surly 1x1 fork , Raceface stem and Easton Seatpost for the bike so for just under $400 I got a pretty well spec'd out bike with the exception of the low end wheels.