In a vacuum I'm not sure he's a "max" guy, but this is an accounting and opportunity game. Certainly he's not worth 2-3x Crowder, but that's also irrelevant.
I don't have the exact numbers handy, but I believe he would make about $21M in his first year so we're looking at roughly $85M for his age 25-28 years. Compare to Horford (30-33) or DeRozan (27-30) for closer to $110M and it's at least tempting. Next summer guys will have the chance to make $30M/year in some cases, so in a way Barnes would be at 2/3 max.
He's a true two-way, multi-position player who can shoot the 3. Maybe not a star, but it sure does fit our mold.
Obviously the target is Durant, but it would sure be an interesting team if we invested in bottom tier (meaning $$ wise) max guys like Whiteside and Barnes, kept our pick, and resigned Sully. We'd be absolutely overflowing 12 deep.
Thomas / Rozier
Bradley / Smart
Crowder / Barnes
Sully / (Mickey / Crowder / Barnes)
Whiteside / Olynyk
bench: Young, Hunter, Bender?, Dallas pick (Maine), euro stashes
Minutes something like:
[GUARDS] Thomas, Bradley, Smart: 25-30, Rozier 10-20
[WINGS] Crowder, Barnes: 30 (24 each at SF, some PF minutes each)
[BIG MEN] Sully, Whiteside, Olynk: 25-30, Mickey 10-20
+ Young, Hunter, Mickey