I think LS1 cars range from about 15k-25k depends on how much work has been done. Basic clean swaps with stock parts still on the car and decent paint....10-15k. Not saying my car is the nicest because there are plenty of Z cars on this site I would love to own....I have done, swap, mods to motor, brakes, suspen, paint, and other odds and ends. I think it adds atleast 5k to the value even if I have 15k worth of parts. So honestly I think $20k for my car is a great deal if you don't want to build one and your taste is close to what mine is but I have to add $5k to that price for me to even think about selling it.
Some of these LSx cars I see on HBZ I think are great deals because all the work and time has been put in. I just started to like the build part of the cars, reason why I started another one. So how much is your time worth?
Jeff is right, I wouldn't take less than 25k for it and I know I have much more than that into it but you don't build cars to make money and if one thought the price was high, I would say price it all out and build =P