You need to provide more input as to what you are going to use the machine for to get a useful response...
One processor or two processor?
4 core processors or 6-core processors?
Video card you are shooting for?
These alone will make hundreds of dollars of difference in the price.
The spread goes from $600 at the low end up to $3500+, with "average" on the non-irrational side being $800 - $2000 depending on specs.
[doublepost=1452549650][/doublepost]To get a better idea go on ebay and look at completed listings, then you can take off a couple/few hundred dollars for a private sale.