How can I offer pro sites discounts or have levels?

I am still in the launch phase of my multi/pro sites project. I am going to be rolling out real estate themes soon. I have already engaged two sponsors. Basically, sponsors will refer real estate agents to my website. In exchange for the referral the sponsor gets a permanent ad and link back on the agent's website. The agent gets a reduced monthly rate.

I know coupons only work on the first payment. Are there any other options, besides setting up a level for each sponsor, that I can use for sponsored users? Is it possible to hide pro site levels? I am thinking through how Membership can be used, but I'm not quite at a solution yet. Any ideas?