I think referrals come because people know that your work is done well and that you are a professional.  If you provide an incentive too me it muddies the picture.  To me, the only exception to this is if you and another business partner have an arranged business agreement that when you refer out to each other then you pay each other referral fees.  That is not uncommon. Otherwise, I think thank you and be open to send them referrals freely is a great way to go.