According to Roush the price of the 428R depends on how much the Mustang GT costs that it is based off of. The 428R package itself retails at $16,999. Assuming the GT is roughly $25K-27K that would put the 428R around $44,000.

Of course it is a limited edition (only 200) so I'd assume the dealers will rape the customer for another $10K-$20K.