"Given the value that we get out of TextMate, it’s already grossly underpriced. Please let us give you more money"

The value for you isn't probably the same for all the customers.

Some people make millions from the work of the Apache Foundation for example but others use their web server to run little websites. Some would probably pay tons of money for it, others won't.

I am not saying that a upgrade fee for TextMate 2 will not be fair, but that is solely at the author's discretion.