Recently, MotorTrend tested a bunch of high performance cars, and presented the results as spiderwebs, with a combined performance number based on things like steering accuracy, brake feel, roadholding, etc. The result was neat, but not practically useful because it didn't factor in cost.
Then Winding Road got in the game, and proposed the Speed/Dollar index, which is HP/weight/price * fudge factor.
A problem with the SpD metric is that it only cares about power/weight ratio, which can favor cheap cars with big engines, such as the Mustang GT, or even a V8 Chevy family hauler. Nice in a straight line, but what if you are interested in autocross or rallying?
I think a better metric would be something like (HP/Weight + Slalom Speed/60)/Price * K, which would add a dynamic handling component--the speed through a slalom course, in this example. Or perhaps a lap time or average speed around a reference track would be better.
Both Winding Road and Motor Trend put the MazdaSpeed 3 near the top of their list, in terms of bang-for-the buck. Now I want one.