If I recall correctly, they paid something like $23 million to acquire the leases (a few parks have been added since then with this structure, so likely a bit more now if they sold them as a package) and then they pay rent to the owner, in this case the company out of KC, EPR I think it is.
I don't believe they own any of the rides or structures....that is EPR who does, so I assume EPR chooses to add new attractions or not and how much to spend. Maybe they just fall on SIX to help with buying in bulk or leveraging any deals with vendors.
Here is from the press release at the time, which is why I interpret it that way, but I may be incorrect:
Houston, Phoenix, Buffalo, and Oklahoma City Parks Expand Market Reach
GRAND PRAIRIE, Texas--(BUSINESS WIRE)-- Six Flags EntertainmentCorporation (NYSE:SIX), the world’s largest regional theme park company, today announced it has entered into a purchase agreement with affiliates of Premier Parks, LLC to acquire the lease rights to operate five parks owned by EPR Properties (NYSE:EPR). The parks have previously been operated by Premier Parks, LLC of Oklahoma City. These latest acquisitions will expand the company’s portfolio of North American parks to twenty-five.
“Today’s announcement represents another milestone in our strategic North-American growth initiative to seek out park acquisitions that expand our addressable market,” said Six Flags Chairman, CEO, and President, Jim Reid-Anderson. “These are all fantastic properties that complement our existing portfolio and provide tremendous added value and cross-visitation opportunities for our extensive Membership and Season Pass base.”