You could set a minimum - "I'll pick a winner when we hit 100 tickets!"
Whatever you're offering, take into account how much it costs (if it's panties or something, plus mailing costs, gas to the post office, etc), how much time you need to spend on it, and some profit. Example with made up numbers, if you spend $20 on panties, $10 on shipping and gas, then you want $20 as profit, you'd need to make 1000 tokens minimum. Say you sold tickets for 10tks, you'd need to tell at least 100 tickets to make it worth it. So you can play with the numbers; selling more cheap tickets or less tickets at a higher cost. Whatever it is make sure you've covered your costs and that you set the rules in advance so it's fair for you and your members.
One more thing, we had a discussion here a while ago -- models often do deals where they'll do double tickets sometimes. If you want to do that, I'd suggest saying it in advance so members don't feel ripped off. Some members said that they felt it was unfair to buy tickets at regular price only to have other members get them for half the price the next day. If you do want to do something like double tickets for offline tips, I'd just put that in your rules from the beginning so it feels more fair.
