Summary
Boban became one of the NBA's most popular characters due to his size, personality, and dominant stretches of play. In his days with the Clippers there were often many people clamoring for Boban to get more minutes, because he was one of the more productive players in the league on a per-minute basis. The reasons against Boban playing more minutes were obvious: due to his conditioning he probably couldn't physically play a high number of minutes, and on defense he could often be a massive liability against certain lineups. If Boban has to defend in space, it's pretty much a lock that he will get cooked, and pick-and-pop bigs can get wide open looks against him.

However, despite those weaknesses, Boban has turned himself into a good change-of-pace backup center that can dominate inside against the right matchup. He has good hands and really nice touch around the rim. His rebounding rate on both the offensive and defensive ends have always been really good. His numbers actually fell off a bit this season, as his once-elite points per shot attempt has dropped below league average for the first time in his career, according to Cleaning the Glass. That is something to be worried about as Boban enters his mid-30's.

I would still be interested in a Boban as a third center, but he probably is overmatched as somebody that can be a full-time backup. Is it more spending more than the minimum on a third center? Probably not, but I can see the argument for Boban getting a little more than the minimum due to his unique skillset.
 
Cap Considerations
Marjanovic will be an unrestricted free agent with a $4.6 million cap hold and Early Bird rights. Using his Early Bird rights, the Mavs can offer Marjanovic a starting salary of 5% above the average salary (around $10 million), which should be more than enough. In the unlikely scenario it's not enough, the Mavs could use cap space to re-sign Marjanovic if they operate under the cap.
 
Other teams will have multiple ways to sign Marjanovic as even teams without cap space could possibly use any of the Non-Taxpayer, Taxpayer, or Room Mid-Level Exceptions to sign him. There's also a chance that Marjanovic ends up settling for the minimum.