Well if the Beeb were able to get Moore for DW, it would no doubt be for something like a Christmas special, or perhaps use him as the villian who causes a regeneration.
But as he sad he's interested in being in either DW or Sherlock, the BBC might be able to get him at a resonable price. For some big name actors producers might think they are out of theri price range when considering actors for parts and don't even consider them. Sometimes the actor has to make the first move and express an interest.
I think that might have been the case with Whoopi Goldberg on TNG, she apprached the producers about appearing on the show, and she was willing to appear at more or less the going rate for a guest star on the show. Rather than an apperance fee her star power (at that time) would demand.
He's only worth a reasonable price at most - he's been trading on Bond for a long time now and most of his recent stuff has been voice work and cameos. He's more of a personality than an actor these days.
I don't watch Sherlock but I have no interest in seeing him in Who.