Today, dollar for dollar, is the 318 faster than the 340 ???

IMO, the cost of the core is the difference, good pistons for the 340 may be easier to find though. The machine work, gaskets, bearings and other parts will be the same. It also depends on how radical the build is, if you're looking for 400 hp the 340 may be cheaper (parts) to get there, you will still have the core costs difference though.

In the end how do you judge performance per dollar? If you doing a typical street type performance build and the 318 will get there by all means go for it.

It would be interesting to see two builds where the only difference is one is a 318 and the other a 340, dyno or et in the same car.

Why not throw a 360 in the mix?