I'm sure a new arena could be developed commercially, perhaps at the Eastlands site (it has no need to be in the city centre per-se). The figure relevant would be the one necessary to fund compulsory purchase of the site.
That isn't of relevance, as the only place where it'd be easy to expand Victoria is the Arena site directly to the north of the station.
The arena changed hands in 2010 for just north of £60m (https://www.manchestereveningnews.co.uk/business/business-news/new-men-arena-owners-post-897239 )
Any compulsory purchase of the site would be for at least this value, presumably.