The actual seat and pitch bit of a stadium is probably the simplest and cheapest bit to do. It's just steel and concrete, with some good acoustic design there's no reason a Kirkby bowl couldn't produce a decent atmosphere (obviously not exactly like Kirkby, that wasn't great). The cost comes with all the other stuff in stadiums, function rooms, boxes, other corporate facilities. We need these really to really push on, but it's something that has to give if money is tight.
It definitely will be interesting to see the plans if/when they come out