Something to consider: good code is an asset, and it's a compounding asset. The better your programmers are, the more likely that they'll produce code that they can build upon, and build upon, and build upon, all with relatively few bugs, so that you can go faster as time goes on and the product grows. Mediocre programmers often produce code that requires more maintenance and slower development as time goes on. Either way, the effect is exponential and not linear, which makes it hard to compare against linear differences in salary. Probably should run an IRR comparison against your cost of capital and decide based on that.
This, BTW, is why companies like Google try to find the best talent they can, no matter what the cost. Productivity compounds, and not always in obvious ways. In addition to writing good, solid code, top developers also attract other top developers to work with them, and they create a culture where high achievement is expected and you can't just slack off. These are intangibles that are really hard to put a price on.