I don't think you're getting ripped. I would consider this for sure. You need a true #1 goalie, and you'll get that. Yes, you're losing Datsyuk, but points can be made thru other players - Ws, GAA, SV%, etc cannot. I'd make the deal and pick up Chara! So, you'd be getting:
Price, Guchi, Grabs & Chara for the Wall, Dats, Berg & McCabe. Do it! And, if you don't: drop McCabe and pick up Chara.
You're still strong in all your positions, and what you'd lose in C, you'd make up for in both D and G. Looking at his team, there's not much more you can ask for that he'd likely give - if it were me, I'd try for another C in place of Grabovski...but I don't really see that happening.
And further - if Chara's on the FA, I'm sure there's got to be other forwards that can pick up the slack of losing Datsyuk. Get rid of Bertuzzi, with his horrific +/- and pick up someone decent. You might wanna see if you can steal a better FA goalie as well (maybe Smith? Auld?)
OR, maybe sub Bertuzzi in for Bergeron? Maybe he'd swing at that? You have a decent looking team - and your forwards can pick up the slack - but you need a tender.