Horse Power Potential of Cylinder Heads

2hp/1cfm for the lift range of the cam is what I use. Meaning if your heads flow 260 @ .600 and 250@ .500 and your cam is .495 after lash, you only have the potential of 500hp in the best of scenarios. That assumes everything else is right enough to not cost any power.
You need to say what the rest of the engine is made up of, and what the budget is. I.e. if your budget is $4K, and you want to run stock exhaust manifolds, "your results will vary".