I've seen a couple used desks being sold without a computer which obviously subtracts a lot. In addition, some older desks don't have recall (TR) which also is a subtraction. A G+ (not just a G) with mostly "E EQ" channels and ultimation can double the price over a similar G with VCA auto. That's what the rock
guys are looking for now.
48 channels and above are in more demand than the 40s, and you'll notice the price difference. Actually, 48 is probably the most in-demand since people are looking for more affordable desks to fit into smaller control rooms or personal studios. You actually don't save all that much compared to getting a 56, but it really depends on what happens to be available at the time you are looking.
A small, older E needing some TLC or re-capping can be found for a reasonable price, but to get something like a 64 input G+ will still cost you some real money.