As far back as 2017 reports had set the cost of building the new Everton stadium at £300M:
Construction News Article: Everton FC lines up deal for £300m new stadium
Guardian Article: Liverpool mayor defends city’s £280m loan to Everton for stadium scheme

On 20th October 2021 the piling process had begun. However, by January 2023 it was being now described as a £760M project. This was now a further steep rise from the £505M, the figure being suggested when work had commenced and an agreement had been made with Laing O’Rourke the building firm to actually build their new stadium:

“On January 12th 2023, speaking in a radio interview with Jim White, Everton’s majority shareholder Farhad Moshiri describes the new stadium as a £760million project, a figure that is some £260m more than the £505m figure for construction that has long been suggested by club officials. Club sources claim that the figure Moshiri quoted is inclusive of every facet of the stadium, from design and planning to construction and even including the potential for further ancillary developments related to the stadium. The cost of construction has not changed and that the contract signed last year with construction firm Laing O’Rourke has not altered.”

As of February 2024, four months ago, it was reported that the cost of the new Everton stadium has now risen to over £800M to build their 52,888 capacity stadium at Bramley Dock:
Mirror article: Premier League confirm real cost of Everton’s new stadium after Farhad Moshiri confusion

Further recent speculation last month on the Everton fan media now states the cost to be approaching £900M

“15 May 2024 — The stadium cost now appears to be approaching £900M, if not actually the magic £1 Billion. That can only be funded from incremental revenues.”

Even at £800M this figure represents a cost of £15,126 per seat.

If those same figures of cost are roughly applied to the building of Casement Park with a 30,000 capacity and not the 34,578 capacity then that cost is £453,780,000 approximately and not the £308M figure that has been recently suggested in the media.

“The cost of redeveloping Casement Park in west Belfast for the Euro 2028 soccer tournament could be more than £300m. An official estimate puts the right total at around

£308m. The figure was quoted in a recent letter by the Northern Ireland Secretary of State Chris Heaton-Harris to the Communities Minister Gordon Lyons.” 2 Mar 2024

On 19th February 2024 , Ulster GAA announced that it would commence necessary maintenance and pre-enabling works for the development of the new Casement Park. Part of this work involved taking soil samples during which extensive toxic contamination was uncovered beneath the ground:
Newsletter article: More questions about the Casement Park rebuild as minister confirms asbestos

On the 12th June 2024 Declan Bogue a Sports journalist during an interview on Good Morning Ulster on Radio Ulster made a statement that there would now be a cost of over £32M, just to clear the site of the huge amount of toxic waste before any type of building work could commence.

The Belfast Telegraph journalist, Declan Bogue made a statement that possibly in excess £32,000,000 would be the cost of safely removing and disposing of the amount of  toxic and contaminated waste which lies beneath the Casement Park ground.

The £32m cost estimate points to a massive cost and  big problem, of safely removing and disposing of such a huge amount of toxic contaminated waste. As N Ireland has no such facility, that now means transportation  to Scotland by road and sea. With an estimated cost of £2000 per container load for safely transporting this type of waste that probably involves in excess of at least 16,000 lorry loads.

What has yet to be disclosed is if this £32,000,000 plus figure had been included in that recent £308M estimate, or is this now a further addition to the total cost of building Casement Park? The total final cost to the taxpayer, has yet to be fully established, but comparing the cost of the Everton stadium, the total cost may be closer to £500M.