To give you some idea of what this attraction will be costing Merlin, I was looking through one of their investor presentations earlier today, and their 4 year CAPEX cycle (peak, low, medium, low) dictates that 2019 should be a low year for Alton, as Wicker Man in 2018 represented a peak year. I'd personally predict that to be around £3-5m, as I'd guess that Alton Towers will get one of the higher budgets in the RTPs operating group for a low year. Not sure if I'm right or not, but I'm sure I heard somewhere that the budgets for each type of year are dictated by the amount of visitors a park gets.
For some reference, CATCF set the park back £8m in 2006, although I'm not sure how much of that would have gone towards the elevators and the IP license. However, the Dungeons probably won't be utilising any new ride hardware, as far as we're currently aware, so the entire budget can go towards theming! That gives me great faith in it to be a good attraction! Also, remember that Hex only cost the park £4m, and look how well that turned out! That involved a brand new ride system, too!