You're right, but at the same time, at a price of £760m (according to Moshiri, if that's true), you would probably expect a 52k stadium to have a closing roof, with all the additional flexibility that would give. It would've been the biggest covered concert arena in the North of Britain. Owners often inflate or conflate the cost of their new stadia with whole project costs, to add value or perceived quality to their investment.
Examples of cost for comparison: Meis design for Roma's 52k proposed stadium was just £300m. Feyenoord were getting a 63k triple tier, on partially reclaimed land with a closing roof for approx £400m. I think costs went up to £500m+ due to increased material/construction costs post covid, so they've put it on hold.
Lille got a 50k triple-tier stadium, with closing roof and folding pitch for <£300m just over 10yrs ago.
The Lille project wasn't just the stadium build though, it was a huge project which included a new stadium as part of a wider project. The whole project was 600m
Romas stadium was never built so not worth comparing and same with Feyenoord.