It's going to be hard to find a decent piston-filler for 50-100. For that price you may be able to find a decently restored Parker 51, which is a great pen. Other inexpensive brands worth looking it are Bexley, Pilot/Namiki, and Aurora As mentioned above Richard Binder's site is great, but you should also check out the following:

1) Pentrace Market Board
2) Nibs.com
3) Thefountainpennetwork.com

Happy Hunting