Any increases that don't involve just standing ratios would require the roof to be dismantled along the east stand and a new tier adding. Not unlike city's extension, the first of which took place in 2014 and cost £50 million (remember a goal stand is not as big as a side stand). I would say it would be comfortably getting up to that 100 million figure.
The cheapest solution would be to add a few thousand by expanding the North stand but we would need to buy the land, the concourse would need to be extended to take the extra capacity and would still cost £50-60 million for very little gain. The club will make most of the money through the premium seats, naming rights, events. Three thousand extra tickets isn't going to make a world of difference especially given the outlay.