Want to have more than 1000 mutli-sites on our WPMU site. What will that cost?

We're looking at a revamp of our current WPMU multi-site http://neighborhoodboutiques.com given some issues we currently have and a desire to create greater flexibility given our vision/mission.

Our current cost investment with WPMU permits us to have 1000 multi-sites at Neighborhood Boutiques. We will want to have capability of at least 7500 each Neighborhood we create. What would our financial investment be at that level?

Thanks much. I LOVE the support we get from WPMU!

Warmly,
Linda S. Fitzgerald