I'm not sure setting it on Earth would really lower the costs. You're talking about a show set hundreds of years in the Future, so, you still need to "futurize" the outside and an office stage/set with futuristic looking adornments isn't going to be noticably any cheaper than a Ship's Bridge stage/set
You'd need some establishing shots in CGI sure, but they could be stock and regularly reused. But something like a window overlooking a park is a window overlooking a park - you'd just need the odd extra in a Starfleet uniform walking past to make it look authentic.
Yes you'd need standing sets - but that's true of any series - but you could combine that with filming in real "future-looking" buildings. Doctor Who for example has used inside of real buildings like The Millennium Centre and the Senedd in Cardiff for future building type filming and was really effective.
You wouldn't continuously need a lot of new CGI stuff all the time like you do on a space show where they visit a new planet or encounter a new type of ship every week.
Some of the political stuff in the president's office in the Undiscovered Country was intriguing, as was some of the Earth based DS9 episodes (like Homefront) showing Starfleet operations on Earth.
Earlier comment about Earth Trek - true, you wouldn't want to mis-sell it - maybe call it "Federation:For the Flag" , "Federation: Highest Duty" , "Federation:To Serve and Protect", "Federation:Prime Directive" or something like that.
Yes it would be a very different show - but it would be something new set in the Trek universe that shed a new light on stuff we've only glimpsed at before.