West Ham can price their season tickets at quite a low price because their expanded capacity has equally came at relatively low cost.
From memory West Ham contributed £15m to the conversion cost plus a rent of £2.5m per annum, for an extra twenty-five thousand seats.
It's therefore in their interest to sell them all and fortunately it's financially viable for them to offer a competitive rate for each seat to do so.
If we're to move to a new 50k capacity stadium then the outlay will be much higher per extra seat, plus the cost of the original attendance.
Increased attendances will subsequently bring additional costs, so it's all well and good saying make it this or that but it has to be financially viable.
Obviously there'll be the potential for sizeable extra revenue from commercial and hospitality as
@The Esk has rightly mentioned.
Yet that'll only cover part of the cost, thus ultimately they will only build enough seats that they are confident to fill at or above current prices.
That's simply why I can't see a stadium around the sixty to seventy thousand mark as if there's empty seats it'll hit the profit margins!
