Look, I'm doing my best to report what I recall of somebody else's very rough outline plans. I'm neither a stadium design expert nor particularly "in the know".
As I understand it (which at any point could turn out to be completely wrong), the whole stand and hotel/conference/car park development at the Park End could be done for in the region of £75m. When you do a development like that, I'm sure there are savings to be made on each part by doing the whole thing together. Rebuilding the stand alone, or building a similar hotel development without the stand may not be as cheap.
As the hotel and corporate dining facilities, lounges, etc are integrated with the stand, part of the construction cost is associated with them. If we want to get to keep any of the income from pre/post-match dining or any other events held in them, we'd need to contribute more than the £25m, otherwise the developer would keep all that revenue. What % we contribute (and therefore what % of revenue we get to keep) would depend on many factors...not least what we could afford and (I assume) how much of a risk the developer perceived the project to be.
To repeat, based on Trevor's presentation, I don't think the stand on it's own can be done (at least to a decent/suitable standard) for £25m, but that could be the club's minimum investment in the redevelopment at the Park End. Of course minimum investment would yield minimum gain.