unfortunately, shelby gt 500's have always been quite up there in the market. a few cars go for more, such as the road runner, and the 69-72 plymoth hemi-cuda (429 cid) sells between 100k, and almost a million depending on the condition, options, and bidder
there is a company that is taking vintage mustang fastbacks, and converting them entirely to new shelby gt 500s, much cheaper then a classic, and about the same
heres a couple websites you might like
http://www.shelbymustang.com/
http://madwheels.com/vidoes/index.html
http://www.geocities.com/eskoufos/shelbyprices.html