First getting into Tau I got a D-Fish and 12 Firewarriors for something like $25 including shipping, and the fish had the Forgeworld Gubbins on it!
The best deal ever though was one that I went in on with a friend. $300 bought a huge lot that was advertised as "Warhammer Figures". We got:
A Tau Army
A Chaos Space Marine Army
A Tomb Kings Army
A Lord of the Rings Harad Army
Carrying Cases, battlefield accessories, bits and bits and bits
Seriously, it was heaven-sent. My share bought me the Tau army. I waited and waited, and finally, on the day I thought it would come I called my friend to see if it had arrived, and he said no. I was super bummed until I went home and found an entire Tau army laid out on my counter top!
(My wife still makes fun of me, apparently I "shoved past her" in my enthusiasm to get to the minis. I maintain that I stepped around her in a perfectly gentlemanly fashion.)
I got about 48 firewarriors, 48 kroot, 3 Hammerheads, a Devilfish, 3 or 4 suits, and a broadside, for my $100 that I threw in. That was really exciting because it was my first army getting back into the hobby.
The only thing was, I've learned since that buying used is a mixed bag.

The amount of cleanup, stripping, disassembling and reassembling almost made it not worthwhile...