Do you think Ullmark will cost a ton? Grzelcyk didn't play that much last season and he has one year UFA, in fact he is defenseman of the third pair. I think he would look good with Boosh. The rights to Clifton are worth next to nothing. I would think something around UPL + Joki + pick maybe.
I think Boston could definitely get a significant return for Ullmark. Assuming he wins the Vezina, he's still signed for 2 years at a very reasonable cap hit. He's only had 1 season that he posted a save % below .913, and he did that playing mostly for the Sabres. He's been extremely consistent, and the only knocks would be his injury history and playoff performances though he's been fairly healthy in Boston.
Edit: I mean significant as far as goalie returns go, since half of them get traded for 3rd round picks.