The newspapers were talking about the full cost of remediating the soil of 50 acres, PlayersLdt excluded costs that don't change whether the soil is contaminated or not, since you need to dig a big hole either way for an arena unlike in the original project where there was going to be green space that needed to be remediated, in a hypothetical dig of 1.5 metric tons to make a point, that's why the numbers were significantly different, but also irrelevant to the cost of this project since we aren't remediating 1.2 m cubic meters of soil, not even close.
Even if you assume the entire new lebreton project (all 10 acres) is at the deepest depth of contamination (16m according to the pic) you'd only have roughly half the 1.2 million cubic meters of contaminated soil, ~205m*205m*16m = 672400 cubic meters of contaminated soil, but that won't likely be the case since the pic clearly shows that there is no 10 acre space that is entirely the darkest orange, more on the actual location later.
The point is you're estimates for 10 acres based on an inflation adjusted price of the total 50 acres project make no sense, the sens won't be on the hook for an extra 150 mil in remediation costs,
As for the location, the original ~6 acres plot was on Albert between Preston and city center, pretty much dead center so that people could go to either station, in the light orange area of the picture you posted. Not sure where the extra 4 acres are coming from, but likely attached either west or east along Albert. Either way, it looks like they'll be avoiding the most contaminated areas.