I waited a long time to buy the WordPress plugin Shopp, but I am currently working on 3 separate freelance projects that need e-commerce so I had to pull the trigger and actually purchase it. Shopp costs $55 for what they say is one-site use (a developer license is $300 = no fucking way). Being a programmer, I planned on buying it and then hacking the code to get it to work on multiple sites when I needed it. Turns out, that is completely unnecessary.
You do indeed need the license to log in to the Support site and to receive automatic updates when a new version is released (although I think I can fix/hack this to work in multiple places too), but you do not need the license for the code to be functional.
I installed Shopp on a WordPress install that I run locally on Apache to test it out, as any sane person would do. Turns out, that was the one install that the $55 bought me, which is ridiculous. So now, that local apache version is my only conduit to all future updates for Shopp.
TextMate costs ~$60 and will let you install it anywhere, any number of times, which makes sense because I use it at home, on my laptop, and at work. Coda costs ~$100 and will let you do the same. All installed instances have access to all updates as well. Seems weird that a WordPress plugin would charge such a premium for code that theoretically could be copied and republished (with only a handful of changes) for free under the GPL at any time by anyone.
I mean, kudos to them for finding a way to charge for code and presumably make a living off of it, I just wonder how many other people have figured out how easy it is to only pay for it only once/or not at all.