Are you in construction? Go get a quote for a building on an open parcel with no schedule restrictions and get one for renovating an arena that includes a severe timeline for completion, periodic interruptions, AND it's located in a difficult to access area with space restrictions on three sides. The basketball/football comment is that there will be a lot of wishlist items that will pop-up during planning & design to make things better for both when the dome was originally designed to accommodate one. "Well, we're already doing A, doing B will only cost a few more million ... etc.). All of that leads to lots of openings for budget expansion. It may be $200m to start, but can be 30% more easily.
The school can't really complain about the costs because they want it done right and fast (which = $$$). Build it off campus (or on south campus) and their self-imposed problems go away, but the overall cost goes up because they'd be building new (but again it's for college sports, it doesn't need to be the Barclay's Center and South Campus isn't NYC). So while it's more expensive to build new, it alleviates a lot of their self-imposed restrictions and would most definitely result in a better building that could be greener, more accessible and accommodating.