Cheers is still TV’s most popular series, but closing time for the Boston bar may be sooner than expected. Negotiations for a 10th season between NBC and Paramount, which produces the show, are reportedly at a standstill. At issue: Paramount’s demand that NBC raise its payment from $1.25 million per episode to a record-smashing $4.8 million; roughly 10 percent would go toward Ted Danson’s salary, which is approaching $500,000 per show. Network insiders insist that the $120 million price tag for a 10th season is too high and that neither CBS nor ABC will pay that much to steal the show away. That leaves the unlikely prospect that Cheers will move to Fox, or stranger still, price itself out of existence — unless NBC or Paramount blinks.