You're probably NOT going to find a brand new cup mute for that price, especially not a good one. My advice would be to check around online for music shops that sell "gently" used mutes. A dent or two isn't usually going to matter - you'll put some in yourself when you get them, and the amount you'll save on them is usually worth it.

Another piece of advice - if you're serious about the trombone, skip the Humes & Berg mutes - they're just plain lousy and they have a terrible sound. Invest the money and get yourself some aluminum ones. You'll save in the long run if you continue to play the horn. Some good brands include Tom Crown, Jo Ral, and Denis Wick.

Hope this helps!