$300m (while maybe somewhat inflated to capture news headlines) is not necessarily out of the question.
Expedition Everest cost $100m and it opened in 2006. Adjust that amount to inflation to today and it's $127m.
Now add to that other points that have been made:
- Cost of all of the theme elements:
- Ruins/buildings
- All of the animatronics (Hagrid, Fluffy, Screwt, Devil's snare etc.)
- Transplanting 1200+ trees
- Highly themed queue
- Combination of ride tech (7 launches, drop track)
- Cost of demolishing and removal of Dragon Challenge
Add all of those things together and I could see it being over $200m