And they got a free new ground off the council
Who do you mean by "they", Brighton or Brentford?
Either way, it doesn't matter.
For Brighton, it was Tony Bloom whom personally bankrolled the entire development. As for "the council", the site traverses two local Councils, Brighton and Lewes, and while Brighton were supportive, Lewes were dead set against it, refusing planning permission and filing appeals, which seriously delayed the project. In the end it had to go to the Secretary of State to overturn their objections.
As for Brentford, the cost of the development was funded by the sale of Griffin Park for housing, plus owner Matthew Benham underwriting just under £100m to get developers on board. If the associated housing development on the GTech site makes top money for the developers who actually built it, then Benham may get some, even all, of his money back, but it's not guaranteed. In any case, the developers will get their cut of any profits first.
Meanwhile, Hounslow Council didn't put any money into it, none. In fact they were initially obstructive, until Brentford fans started putting up single-issue candidates in the local council elections, attracting enough votes to scare the Council into a u-turn and supporting it, granting planning permission etc. Still no money, though.