80/20 Rule

First start with what do you really want. Big difference between a drag race car, drive anywhere anytime car, road racer. I always start with great machining. The things that matter the most are: round, straight cylinders; rings that seal at WOT; and great valve job. After that head flow is first, cam and matching components, ignition, intake and carburetor. It is the combination and tune that makes the package. Compression is dependent on the available pistons, cam, and gas to be run. As far as percentages between cost and performance, not sure you can get there. Some people will spend BIG time and money for that last .2 of a second. However you can always spend all kinds of money and go the same or slower. People do it all the time.