Although I've not always offered prizes for players who test their speed at my game, on most occasions I do. I generally charge 5 trys for $2.00 so prizes should never dig too deep into profits at an event. The question is whether to get standard blow up prizes typical at most carnivals or more costly Pro Sports Licensed items.
So how do they win you ask?
Guess Your Speed
The most common method is guess the speed of your pitch or kick. If I offer 5 trys then the first 4 are strictly for curiosity and getting an idea of their potential. On the 5th try I tell them to guess their speed before they throw it. The guess must be exact. Baseball pitching is a bit easier to guess than soccer kicking so factor that in to your prize inventory. Some speed booths get very, very inexpensive inflates and hand them out to every player- win or lose
This guess your speed method works much better than giving prizes for breaking a fastest speed because it doesn't alienate smaller or weaker players from potentially winning a prize. I usually have a dry erase board lined in age groups to list the high speeds and this I change when a speed is broken.
Raffles:
If you do happen to have a more expensive prize, a raffle is a good approach. When I run kick timing at the National Soccer Hall of Fame tournaments I sometimes offer up one of their soccer balls as the prize. It's a stunning looking ball worth around $25. If a player can beat a speed in his age group or guess their last speed then they win a raffle ticket. At the end of the event I draw a ticket. Even if the player isn't present I have an address or phone in which to arrange mailing the item. If you are using the booth as a promotional tool for a company or organization, this is a great way to give out an impressive prize like a TV, Sporting Tickets or even a Car!