Summary
Forbes's main skill has always been his outside shooting. He's a career 41% three point shooter, and he shot 45% from three this past season. He's not a true point guard in any sense, as he's not much of a playmaker or facilitator, and at 6'3" it makes it somewhat difficult for him to play next to smaller point guards. That's one reason why Mike Budenholzer mostly staggered his minutes with DJ Augustin early in the year, and had one of them playing with a bigger guard in Jrue Holiday.

Forbes isn't someone that can rise and fire in really tight spaces, but he's also not just a standstill option either. You can run pindowns for Forbes and expect him to knock down shots on the move if he gets some space.

Forbes has mostly been on minimum or close to minimum deals throughout his career, and I don't expect that to drastically change anytime soon, but I would be willing to give Forbes slightly more than the minimum. Forbes hasn't shot as well in the playoffs (38% from three), so I don't think he has greatly helped his market in free agency.
 
Cap Considerations
Forbes has a $2 million player option for next season, and if he declines that option he will be an unrestricted free agent with a $1.7 million cap hold and Non-Bird Rights. If Forbes opts for free agency, the Bucks will be somewhat limited in their means of re-signing him. Since they will only have his Non-Bird Rights, the most they could offer him using the Non-Bird Exception is a contract starting at $2.3 million. The Bucks will likely have the $5.9 million Taxpayer Mid-Level Exception available to re-sign Forbes if $2.3 million isn't enough. However, the Bucks could be well over the luxury tax if Jrue Holiday re-signs, so re-signing Forbes could cost them a lot in luxury tax penalties.
 
Other teams will have multiple ways to sign Forbes as even teams without cap space could possibly use any of the Non-Taxpayer MLE, Taxpayer MLE, or possibly even the Room MLE or Bi-Annual Exception to sign him. There's also a chance that Forbes could be had for the minimum.