That's not an average; it's a range. And a preposterous one to boot. Who the heckers is going to spend $250 million on a game? If you get 10 million sales at $60 a disc, the amount the publisher would get would likely just cover the cost of the game! And if the game prices go up, the number of buyers will go down. Maybe, at a push, episodic content created at $250 million over a period of 15 download episodes, would goggble up that much money on a single product.
Which doesn't explain anything about the average cost next gen. I imagine it'll be not much different to this gen, because if spending more doesn't yield more returns, no-one will do it. Even if the hardware is capable of rendering assets that took $100 million to make, it'll only get to show $20 million of assets because that's all anyone will spend. A lot will depend on growth of the gaming market and how factured it becomes. Corporate budgets on the whole will be better invested across multiple diverse titles across diverse platforms, rather than blown on a couple of epic titles.