How much is an autograph from Casting Crowns' Mark Hall worth?

It depends on what is autographed and whether or not you can still buy the item (autographed or not).

If people are unable to purchase the item, it would be worth alot more money than an item that is still available to buy.

Also, an item that is autographed, and that is still available to purchase, is worth more than the same item that is not autographed.