This decision was expected from Ellis, given players usually opt from guaranteed money whenever they can. It has already been reported that the Mavericks are not planning to bring back the mercurial guard, who struggled in the second half of the season and demonstrated attitude problems.
Ellis had until June 24 to make his decision, per terms agreed upon in his contract.
Once the team declines Ellis' cap hold, they will have sizeable cap space to operate with in free agency. Dallas only has five players currently under contract: Dirk Nowitzki, Chandler Parsons, Devin Harris, Raymond Felton and Dwight Powell. Our own Hal Brown wrote more about the Mavericks' free agent possibilities in a huge free agency primer. I'd highly recommend you read it.
You can also check out the accompanying flow chart that goes with it.
If this is the end of the Monta era in Dallas -- and it appears to be -- I can honestly say I enjoyed it, despite the up and downs. We'll have more reaction to Monta's decision and plenty of free agent coverage over the coming weeks.