There are new 6's and 6A's coming on line all of the time, many, like mine, with glass, AP, etc....
All things equal, engine, prop, avionics, construction quality, age, etc... there is little value difference between the 6 and 7's. Each has some minor plus's and minus's with most plus's on the 7's side. The 7's can take 200HP, have a higher Vne, more fuel, more baggage, bigger counterbalanced rudder, and "can" gross higher. The 6's roll faster, primarily due to 2' shorter wing span, and can be built slightly lighter, although most aren't. The 6's design was hand drawn and I think they are better balanced visually than the 7's which share a wing and tail brought over from the 8. It is suttle and probably subjective.
I think it is funny that one always thinks what you own is the best, as I would not trade my 6 for a similarily equipped and built 7 even with it's advantages. I have flown both and prefer the feel of the 6. If you want a slightly more stable machine, then the 7 is marginaly more docile.
Keep in mind, the 7 was born to take advantage of the Cad machinery and new construction methods used on the 8. Same tail, wing, and many other parts I believe are shared, or at least the manufacturing methods are the same.
I agree with Alex that much of the percieved price discrepency is due to age and available aircraft in the market. They are both fantastic machines as all of Vans products are.