Okay, so this takes some nuance.
Pound for pound, yes, the Show Set Installation for Tiana's Bayou Adventure cost more for an install in an existing facility than it would for install in a facility that's being built new for the ride. Being able to control the workflow from scratch and not having to build everything essentially like a Ship in a Bottle is cheaper in the abstract for that specific line item. It is fair to say that's generally true for basically any sort of construction project. Here, things like the Finale structures could be constructed in place and have the building built around them as needed, rather than having to be designed to fit in pieces into the existing envelope without damaging either what's already there or what's going in and being as strong once assembled as they would be if it hadn't had to fit through predetermined openings. Generally that line item is cheaper for a new ride than for a retrofit.
The Riverboat for Splash was constructed in place and the building finished around it - it would have been more expensive to build the building and then build the Riverboat within it, which is why they didn't do that. That's just one example of being able to dictate the order of your work processes for optimal expenditure. Lots of choices like that were off the table for Tiana because the building was already there.
HOWEVER, the Splash Mountain facility is extremely complex and is itself expensive to build - so the separate line item for building such a facility structurally is not insignificant. Same with the ride system within it, and anything that already exists and was held over - foundations, hardscaping, landscaping, rockwork, queue buildings, gift shops, restrooms - not having to build those things from scratch IS an overall saved cost from Tiana's. Beyond the work that went into general refurb of those elements, that stuff was gifted to the new ride by being there already.
Say Disney were to built Tiana's Bayou Adventure from scratch somewhere else, without an existing Splash Mountain to overlay onto. Yes, there would be parts of the budget that could be shrunk by the ability to follow a more natural order of operations for construction and installation of all the equivalent show and control elements that were just installed in WDW's TBA. But it's not like that savings it so great it outweighs the cost of having to build the building from the ground up, you know? So the distribution of the money within the overall budget would shift, but the overall budget would still have to be larger.
You're not getting TBA from scratch for less than it cost to retrofit into Splash. You'd just find that some parts of the budget could be streamlined.
