Not including new hospitality suites, £200,000,000 for an extra 8000 seats equates to £25,000 per seat, which is insanely expensive!
The Hill Dickinson (which is supposedly a state-of-the-art 50000 seater) cost the equivalent of £15,000 per seat, if reports of a £750,000,000 build cost are accurate. Being built on the waterfront would have no doubt increased the costs significantly.
Taking into account the above, £600,000,000 doesn't seem unreasonable to be able to build a 50000 seater, even when considering the higher land cost in south London compared to Liverpool.
Of course, it's not my money, but at £12,000 per seat for a brand new stadium for the future, it has to be a no-brainer!