You cannot trade BOTH of our right handed Dmen.
You could trade a roster player and a prospect who was far away from making our team (Samuelsson, Dumoulin, Harrington, even Pouliot) and one of our roster players; but Bortuzzo is the guy who would replace Letang; just the same way Despres is the guy who would replace Orpik. That would sap our depth for this year, which is not a smart move, and we know Shero (thankfully) wouldn't do that.
Value-wise, though, I don't think we need to add. Callahan is a great player, but his value isn't nearly the same as Letang's. Kreider does bridge the gap significantly, and maybe completely. So I don't see why we would need to add much, if anything. Maybe a 3rd rounder or a B-level prospect (Samuelsson, Kuhnhackl, etc.), but definitely not more than that.
But that package of Letang + 3rd and/or B-level prospect for a signed Callahan (only if the salary was reasonable) and Kris Kreider would be along the lines of what I would consider both good value and a good hockey trade for Pittsburgh. I do have reservations, however, of giving Callahan anywhere north of $5million (aka James Neal money), but I would do it if Kreider was included because he's still cheap, and ONLY IF there was NOT a a full NTC. Because there would come a time, 2-3 years from now, when we would have guys like Neal/Arichibald/Megna/Bennett/Kreider/Maatta who would all have to be signed/re-signed/ready to play, and when Callahan's game may start to decline. If we didn't have an out and we gave him 7 years, we could get screwed down the road in a serious way.
But minus Bortuzzo, and with something a bit less in his place, it's actually a great proposal: a rare thing on these boards and it's a new idea (not a regurgitation of Kuelmin/Stewart/etc.), and a really solid proposal all around.