Yes it does seem a steep price to pay for a #7/#8 defenseman. O'Gara is not a huge loss because he was blocked by better prospects. But Sweeney does seem to like to overpay for low end depth pieces.
I think others would agree that DS drafting has been good, but he has yet to pull off a franchise improving trade. He has a long ways to go compared to the likes of Bill Belichick and Danny Ainge. Even Dave Dombrowski has been bolder with the Red Sox, making moves like acquiring Chris Sale and then signing JD Martinez this offseason.
Sweeney? Since he took over on May 20, 2015, he has traded for the likes of:
Jimmy Hayes
Zac Rinaldo
Sean Kuraly
Lee Stempniak
John-Michael Liles
Drew Stafford
Not exactly an inspiring list of names. And the only player from that list still remaining with the main team is Sean Kuraly, a 4th line player.