The league will modify all contracts if they go to 18 games. They did so when it went from 16 to 17. So no need to account for that.
To get Brown’s updated value for 2024 we can take OTC’s valuation of $10.17M for 2023 and add 13.6% (the cap increase). That’s approximately $11.5M. The average yearly cap increase is about 8% if you want to account for that. That’s about $12.4M in 2025, $13.4M in 2026, $14.5M in 2027 and $15.6M in 2028. IMO that is probably pretty close to his actual value.
But I think you are correct about what he’ll get. Quality OTs don’t hit the market very often and they get paid when they do. The choice is really between him at $15M+ a season and going the journeyman route and/or drafting a replacement. I can stomach the $15M AAV if the guarantees aren’t too bad. But if he gets anywhere near the $18M AAV mark, then I’m letting him walk. I think the Bills probably pay him though. They’ve cut back on OL spending overall and are only really paying Dawkins now. Possibly that is in preparation for Brown’s contract extension.