At the end of the day it comes down to what metric matters most in your business. I don't have enough info about your business model and products to give definitive advice but you should only pay a referral commission for referrals that move your business forward. You are on the right track by only paying comissions for people who are referred, sign up AND make a purchase, rather than just sign up.

Facebook is an example we can all relate to. Zuckerberg has always only reported ACTIVE users, not registered users because ACTIVE users are what make Facebook valuable.

Whatever it is that will make your company more valuable whether it's users, usage, revenue, profit, volume of sales, frequency of sales, etc. is what you should incentivize.

Hope this helps, let me know if you need any clarification. Here to help!

-Michael
@MichaelRHunter