Chris Hayhurst Crash pads are like gym mats for boulderers - they help prevent injuries if you fall off the rock. If you're serious about bouldering and value your safety, you should own one. Used pads cost significantly less.
Prices depend mostly on size and durability.
Look for a pad that can be folded in half for easy carrying. Good ones often include sewn-on handles and padded shoulder straps that allow you to schlepp the pad like a backpack and use it to carry gear like shoes and a chalk bag. Make sure the pad is durable and can hold up to repeated falls and abrasive rocks. The inside should consist of layered foam and the outer shell should be rip-proof. Buy a pad that makes sense for you.