To answer this question you'll need to know the lifetime value of an affiliate that joins your program. Once you calculate the value then you'll be able to determine what you can afford to pay.

I would also look at qualifying what constitutes a valid referral to your affiliate program. The lifetime value of the average affiliate who joins your program is of little value compared to the lifetime value of an affiliate who joins and driving traffic in the first 30 days. Or better yet sales in the first 30 days.

When you calculate the lifetime value of :
An affiliate eho drives traffic in the first 30 days or;
An affiliate who drives sales in the first 30 days you will find that the value is significantly higher (so you can pay more for these referrals)

I would not pay for just joining the program. Up to 90% of these sign ups may not drive traffic. Another concern is that some affiliates may incentivize sign ups, for commission, so you will want to specifically prohibit incentivized traffic.

Keep your criteria for a valid sign up ( # of clicks or # of sales) confidential so affiliates do not exploit the system.

Hope this helps.

Mick