I'd easily pay $100 for a perpetual license for a client that has as many capabilities as TB, plus does exchange calendars/address books right. I'd expense it then, of course :) And it would be worth every cent - while TB has many excellent features, lack of some more enterprise-y things probably cost me much more than $100 in time and effort over my career.
The thing is, however, $100 paid to closed-source software vendor never guarantees he won't be out of business or bought by some Novell that will promptly run it into ground in a year or two. With open source chances for product survival are higher - though, as we see here, are in no way guaranteed.
Just in case anybody cares, I'm currently on Mac OS X, but when I worked on Linux & Windows, I used Thunderbird too, and the same applies for both of these OSes too.