Are you looking for U.S. coins or just anything (worldwide)?. If your looking for world coins just peruse Ebay listings. I recently got a 1700 6d Venice copper for $12.00.
There are a ton of world coins are easy for under $100. As rdwarrrior asked, can you be more specific about whether you're looking for U.S. coins or just anything (worldwide)? Or perhaps Pre-Federal U.S. or U.S. Colonial?