B&K 707 is a good one, although it'll run slightly more than the $100 you mentioned. One thing to watch out for, I've been told, is that if you get a tester that handles both the output and input tubes, you want to get a tester that won't put too much juice into the 6DJ8 types (the B&K fills the bill there). I'm sure there will be some other suggestions as well.
