Also, they already have a rather large cave (a cold war bomb shelter, if memory serves me correctly) to build in. No need to erect walls or a roof, no big deal with electricity, fire safety or plumbing (everything is installed already, but might need to be rerouted), no outside themeing (the pre-existing dragon head is all there is to see of the attraction from the outside).
Not saying a ride like this comes cheaply, but they already have a lot of infrastructure in place, and that might bring the cost down a lot.