Unless he establishes himself as a legit starter capable of consistently playing 25+mpg, I don't think there's that much of a market for backup Centers like Williams
Who are his comparables? Nerlens Noel maybe? A guy on his 3rd contract his 3rd contract has yet to crack more than $5m in a season.
As of the current market, you can go the established vet route and sign Whiteside, Cousins, Howard, Marc Gasol, McGee all for under $5m.
You can go the reclamation project route and try your luck with Noel, Okafor, Cauley-Stein, Len, Chriss, etc, again all for under $5m, most under $3m.
Then look at most of the Centers Ainge has signed in the Stevens era outside of Horford: Theis ($5m), Kanter ($4.8m), Baynes ($4.3m-$5.5m). Sure Ainge will sign some Centers for more (Thompson ~$9.5m, Amir Johnson $12m, Zeller $8m), but that's more to have the flexibility to use their salary for trades then it is for their production (and why those are always short term 2 year deals usually with team options/non guaranteed money).
I like the guy, would love to see him succeed, but don't think there will be a huge market for him. Don't see him getting an extension or a long deal.