Well, since the dollar is declining in value vs the yen, it's probably going to be tough to find one under $100 U.S. with shipping. I picked up mine off eBay for $125 from a Japanese seller which did not include shipping in 2007 and he let me add a couple of games for no additional shipping cost.
You should definitely check out risingstuff.com, like OldSchoolGamer mentioned. The few FX-systems I've seen on eBay have gone up in price over the past year. There's one seller there who has had the same group of FX games listed for quite a while at very high prices.
The RisingStuff guys are great and they've got a big selection of games at prices that beat eBay. I picked up 3 FX games from them over the holidays and they were in excellent condition. Their shipping charges are pretty reasonable, also.