If it stacked up they'd have built it wouldn't they. We know they looked at it and we know they didn't build it.
Brighton generate £125 per fan more than AFCB did in the PL, Southampton get £90 more per fan. Everyone else either gets about the same (Watford, Leicester, West Ham etc.) or significantly lower (Wolves, Cardiff, Palace etc.).
Brighton also get over 30,000 who pay these prices. We would need to treble attendances whilst increasing prices.
You mention new build projects but they would also factor in risk. Eight years payback period would be a low risk scenario where your income is contractually guaranteed for that period plus a healthy prospect of it continuing at that level. What's the matchday income on relegation?
Also the return of the investment is the difference in income compared to now. £5m is what they make now and will continue to make if they don't invest £100m in a new ground. Outgoings are £400k, not particularly onerous. Running costs of the new ground, even if owned, would probably be that business rates in a 25k stadium would be much higher.
What could a new ground generate? £10m? £12m at a push? So it's £100m investment for £5m-£7m increased income that is risky as you like.
As I say, they looked at it, did a bit of market research and didn't go for it.