Just because you have multiple parks purchasing from one manufacturer doesn't mean there isn't an opportunity for a collective discount. Aggregate purchase orders can be arranged with multiple buyers for one seller.
On who makes the purchase decision, it is often a top-down decision. Individual parks are indeed cutting the check for their respective projects, but it is often the corporation that decides which parks add what rides at what time. In Cedar Fair's case, their Senior VP for Planning and Design, Rob Decker, leads a team of Cedar Fair-level employees who head design efforts for each respective park. In this way, individual parks report up to corporate level, which in turn has authority flow down on project decisions. Cedar Point didn't decide to build Gatekeeper, Cedar Fair did. Kings Island didn't decide to build Banshee, Cedar Fair did. Carowinds didn't decide to build Fury 325, Cedar Fair did.
The advantage of having a top-down approach to design is it allows for a better continuity of ride experience across each respective park, and provides for efficiency gains in cost and time. Corporate-level planners are in charge of all design work for all parks, meaning less volatility across each respective park. You can see this in a number of ways: building marquees, roller coaster stations, marketing advertisements, etc.
On cost of rides, it will be difficult to prove any position, as no one knows the true cost of these rides. That is, I don't know what the cost of a 300 ft. roller coaster is - there could be discounts, and there could not.
It is also difficult to compare the cost of roller coasters as apples to apples - CPI inflation, extraneous project costs (land prep, etc.), and other factors means no two roller coasters will necessarily be priced the same.
However, there are very fundamental economic principles, such as aggregate purchasing discounting, that while wouldn't spell a wind fall of cost reduction for the parks, could still be playing as an effect. On the flat ride side, this type of purchasing order is most likely at play, such as with the Wind Seekers and Larson Loopers that you pointed out.
