Huh. Turns out the ATTiny45-20MU is around 30% cheaper than the ATTiny45-20XUR, depending on how many you buy. Even buying them in a tube doesn't save you the kind of scratch that the QFN package does. Even if you were to buy them a reel at a time, the QFN package is 20% cheaper (of course, then you're buying 5 or 6 thousand of them).

Well, it's all the same to Bob, of course, so I'm going to try a board with the QFN part and see how that works out.