ACU Metal Fab makes them for what seems to be a decent price, but they aren't $20.
I got this quote on 13 Jan 2009: 7" x 12" x 3/4" with 5" cut-out, removable perforated screen, center drain, s/s #4 brushed finish $86.22 ea
The 5" cut-out is for a 3" tower as it fits around the base rather than directly against the tower. If you want it with no drain, add $20. If you want it flush mount, add $20. So +$40 for no drain & flush mount. Polished finish would also add to the price, but I don't know how much.
Most other SS drip trays I've found have been $100+ and I've heard only good things about ACU (I got my tower from them), so they're probably the best bet. I was about to order one from them, but seeing this one for $20 is making me rethink dropping $120 on one.