LoR and their subcontractors started manufacturing elements for the stadium a long time ago, well before they were ever needed on site. The cost of all major elements of the build would have been fixed so the club wouldn't be on the hook for any sudden jump in steel prices for instance. There might be other instances where things fluctuate in price but I can't think if anything really that would change so much that the club would need to fork out extra and even if they did it would likely be nominal in the grand scheme of things.
The club and LoR would have been through a period of Value Engineering before finalising/ signing the main contract so prices for everything, including any changes in spec, should have already been agreed.
There is always potential for unforseens and something might occur where the availaibility of a product changes which could mean a more expensive alternative is needed. However, this is unlikely and we wouldn't be talking huge sums anyway.
Therefore, the only potential significant changes in the contract price would come as a result of changes requested by the club. So whilst the contract sum won't be necessarilly 100% fixed, it is as close to being as it possibly could be. The main point being the club shouldn't be at the mercy of a huge increase in build cost because the price of steel or concrete suddenly jumps again.