What would be an acceptable offer for Parsons in your opinions?
I think it's more about what Dallas thinks is acceptable. Not being flippant here.
Had DeAndre Jordan not reneged on the Mavs, things would not look so precarious in Big D. [See the rumored consideration of pursuing Larry Sanders.] Now, their entire season, and perhaps Dirk's sunset, rides on whether Parsons and Matthews (and Deron Williams) can approach former levels of production.
I think trading Parsons would be an option for Dallas under only two scenarios: a total tear down/rebuild, perhaps even involving trading Nowitzki; or a sweetheart return that gives Dallas help elsewhere (at center, probably) without a significant dropoff at SF.
I can barely envision the first scenario. Dirk is a Mav for life, and I can't see Cubes abiding a rebuild.
The second scenario? I doubt anyone would be willing to make the kind of overpay that would make a Parsons trade palatable for Dallas.
I think Dallas has to look at other ways to strengthen the roster through trade, and hope that they stay healthy, that the aforementioned players produce, and that role players exceed expectations.