Also, if you buy a Mac that's a few years old, you still get a great computer at a low, low price. I picked up an 11" air for under $400 off eBay and have used it to build a startup that supported me and my support person. Ran dev tools fine, ran browsers fine, and even though it was years old the battery life was fine.

You don't need the new hotness to get good and useful work done.