How do i determine the value of a francise.?

I am interested in purchasing a francise, this will only be the second unit, of this particular business and the first francise. There is no nationwide recognition or advertising, only local, and a small amount. What is a good rule of thumb to determine the value? This particular francise asking price is $50,000 with 6% residual on gross sales. The original business gross sales are approx. $500,000-$600,000 I dont feel that i am getting very much support from francisor other then initial assistance with set-up. Vendors names, product training etc that would require 6% residual on gross sales. My prospective location of this francise may produce gross sales of around 1million, but still think $50,000 and 6% is to much. 1 million in gross sales would be basically a break even figure. So if anyone can assist me i would greatly appreciate it.