Estimating quantities for instant prizes
Determining how many instant prizes you will need for a game is not too difficult if you have some idea of how many players you expect to have at your carnival. We covered this topic under in Planning (see Determine Your Budget) but it is worth repeating here. If you know how to use a spreadsheet (like Quicken or Excel) it will make your work much easier. I have an example Excel spreadsheet that I can email to you on request.
Determine how many players can play the game during your carnival You could make a chart like this:
If our carnival is 3 hours (180 minutes) long, 

And the game takes this long:  Then this many can play: 
30 seconds 
360 players 
1 minute 
180 players 
2 minutes 
90 players 
3 minutes 
60 players 
Do a reality check here  If you are only expecting 50 players at your carnival, you probably won't get 360 players per game, unless it is so fun they want to play it 5 or 6 times!
Out of the possible number of players, how many will win at each level? That is, if you have a game where the player throws three beanbags, how many will get one, two, or three down? This is somewhat of a guess on your part unless you've had this game at your carnival before. You could make a chart like this:
If 180 players can do this game in 3 hours, 

Then at this level:  I expect to have this many winners: 
Level 1  One hit on target 
84 
Level 2  Two hits on target 
60 
Level 3  Three hits on target 
36 
Come up with target cost for prizes for the game. You could make a chart like this:
If 180 players do this game, 

And this many win:  And prizes for this level cost:  
Level 1 prize 
84 winners 
$0.05 each 
Level 2 prize 
60 winners 
$0.10 each 
Level 3 prize 
36 winners 
$0.30 each 
Then this game's prizes will cost: (84*$0.05)+(60*$0.10)+(36*$0.30)=$21.00. 
Cost of prizes for the game / Number of players = Cost per player
So for our example, it would be $21.00 / 180 = $0.12 per player. So as long as it costs at least $0.12 to play the game, you are going to come out ahead.
Plan to order more than you need. Realize that the numbers you come up with here are only estimates. Order more than you think you will need to give yourself a cushion.
Choose your prizes. This is the fun part! Go through catalogs and look for prizes that are close to the amounts you want to spend. In my example, I would look for prizes that cost around 4 cents, 10 cents, and 30 cents. Remember that you just have to average these amounts, you can go over and under as long as you come close. Have a few choices, your first choice may be sold out.
Check your work. After your carnival, take an inventory of your leftover prizes. Did you run out of items? Did you have tons left over? Write it all down for next year's carnival. This will help tremendously in the future.