[olsport]

Many factors determine value (as with anything collectible). Pictures of the cap would help alot. Here are a few of the many factors to consider:
What condition is it in, from like new to beat up?
Was it store bought, or a Boston Red Sox player's game used cap? A beat up store bought cap could be an item nobody wants for any price, all the way up to a game used cap that Ted Williams used in a game in great condition selling for thousands of dollars.