Actually, yes, it does make sense. It's been stated that they want to make greater use of the stadium than just playing football matches - eg trying to put on concerts, making it a conference facility, etc, etc. The bigger the stadium, the more space there is to hold these events, and more people can attend at one time. Increased cost of the build means the development can be used for more things which in turn means more profit.
But if he was out for profit he would just come in build a crappy stadium and then leave.
He wouldnt be trying to stay for the long term and the point of those extra revenue streams is to fund the club, albeit thats never been really clarified.
Even with what you say the stadium could still be a lot cheaper to build and if he was really just trying to solely make money from us in the evil villain way that many now describe him then he wouldnt have been so ambitious.
Yes he wants to make money and that will be a key motivator for him but Im totally convinced the football side has won him over.