Kind of defeats the principle of referrals when you refer someone you don't know for a bonus! If you truly have a good recruiter, just post their name.
Parallon is a combination vendor manager and agency and is owned by HCA. They have at least 100 subcontracting agencies who often pay more than Parallon the agency. So calling them the exclusive agency is a bit of a stretch.
So no help in cutting down the number of agencies to shop for the original poster. But you need to be signed up with several agencies anyway, so start calling them and learn which ones are best for you. Yes, pay rates vary significantly based on geography (California usually pays better than Texas), specific need, specialty, experience, and agency. Two agencies at the same hospital could indeed pay differently. Only way to find out is to shop around.