It might be easier to answer the question " how much $ per hp after 650 hp". Should be easy to get to 650 or close, then start adding up all the expensive stuff and related gains.
Max compression. Gasports. Paper thin rings. Vacuum pump. Custom windage reducing oil pan or dry sump. Max lift rollercam and attending valvetrain. Custom ported intake. Custom stainless headers.
An example is for my 550 cube 440-1 motor that made 926 hp. I bet i have $13,000 just in heads and valvetrain alone. To build a a high hp shortblock cost is over 12,000 for parts! Sure, there are cheaper ways to get where we want to go, but a lot of us go down a limited road(240 head on a 512) and paint our way into an expensuve corner. Consider the bigger head for power over 600. Things will be cheaper for hp past about 650 by a bunch.
