It was 50 million like 8-10 years ago.
I don't think that number applies. 8-10 years ago they didn't have an arena site so who knows what the old hypotheticals applied to. Could be more, could be less.
Also, one thing people need to keep in mind is that the decontamination costs are only those above and beyond the excavation and hauling costs that would be assumed for the project regardless. So if it was $50M 8-10 years ago but that was from scratch, the actual cost for decontamination now that there is the need to excavate comes down to just the additional tipping or incineration fees and potentially added hauling costs depending on where the contaminated fill disposal site is.
It's not quite that simple but point is a huge percentage of the cost is now assumed in the need to excavate regardless. This doesn't necessarily favour the Sens unless the NCC says they will assume 100% of the costs to decontaminate and they transfer over the open hole. That will 'save' the sens 10's of millions. But as others have pointed out, it all comes out in the wash in the final sale price.