You originally said (incorrectly) that a new stadium was not our first choice. Now you fall back on a 2nd line of attack.
At the time, the option to build over WHL didn't exist, but things changed. And with that change, it was preferable - for obvious reasons - for the club to stay in its current and historic location, where its roots lie. I imagine that would have been the choice originally, had the option then existed.
You claim to know about the relative costs of the two options, but I doubt that you actually do ... especially when taking into account today's prices rather than looking back several years.
It's also the case the new stadium complex has many design features that were not part of the plan for the Olympic Stadium - the capacity to stage NFL games for instance (with dedicated NFL facilities, including an entirely separate pitch, that create the future possibility of an NFL franchise taking up residence). I also wonder if your £250m figure included the hotel, apartments, extreme sports centre, housing and many other features that are part of the current project and which, whilst pushing up the cost, also push up the potential financial returns.