This can be tricky.

Generally I recommend paying commissions on revenue-generation events only. The reason being, if it is free to generate a lead then it makes it really easy to take advantage of the system.

That being said, there are some ways to go about it. Calculate (you should already have this) your cost of acquiring a lead (CAC). That's your baseline. You can now set your referral commission at or below that CAC number. Since you only pay a commission based on the generation of a new lead you guarantee a positive ROI.

A prize/sweepstakes is not a bad idea. Make sure to create a prize that really resonates with your target demographic (YES: set of All-Clad pans, NO: an iPad) I would run the program the first time based on your best estimates; if you expect to generate 100 new leads, and would normally pay $10-$15/lead then you can afford $1,000-1,500 prize.

You can then adjust the process during future iterations based on:

1) The quality of the leads you received. Referrals are generally more valuable, but scam/spam leads are not.
2) The number of leads you generated.

If you want to work through details in your specific case let me know, I'd be happy to set up a call to discuss further.

Good luck!