Best prices I've seen are at crate engine depot.
There are two diferent prices, one for replacement engines (i.e. STOCK) and there is a premium for GM Performance engines.
http://www.crateenginedepot.com/stor...P9056C274.aspx
Reman LS3 - $5300
http://www.crateenginedepot.com/stor...0-P1897C0.aspx
New complete LS3 - $6300
Just depends what you are looking for I guess.