Look at the current roster and how it's set up. Getting someone like Mankins with Davis and Columbo's money ( after they're cut ) is a great idea and absolutely the path this team should take.
Trading a top 10, or top 5 pick, for two #1 draft choices and proceed to draft 2 O-lineman in this draft is NOT the path they should take. You mentioned Costa not playing bad. I'd say he did better than that. We still have Sam Young and Robert Brewster, plust Travis Bright, who we are developing. Spending two #1 picks on O-lineman in this draft in particular would only add to the gluttony of developmental players we already have.
You want to know why I mentioned Robert Quinn above ? It was because my first idea was to check out Anthony Castonzo, BC's starting LT. I, like you, thought of the Patriots and the possibility of moving down and even though we would be out of Patrick Peterson's range, we could pick somebody like Castonzo and CB Ras-I Dowling, for example, or another DB who might have made a late move into those range. Well, I saw films on Castonzo. I like him, I think in time he could be a terrific LT as he gets stronger. He's mean and he competes. BUT he's not ready to start in the NFL next year. Quinn got the best of him the majority of the time, although Castonzo did get his shots in and also looking at the FSU game, it showed that Castonzo has problems with speed rushers and upper body strength at this time.
We already have two of those developmental tackles. We don't need a 3rd one.
If the Cowboys end up with a top 10 pick, they must select a player who's going to go in that range, regardless of need ( unless is a QB, WR, or TE ). As it so happen, and as of right now, they could get a very good one that's not any of those 3 positions ( wether it's Peterson or Quinn or even Karrigan ). They cannot pass on a possible impact player for the sake of need.
On a side note: I'd stay away from 1 player who, imo, is way overhyped: Nebraska's CB Prince Amukamara. I'm a Husker fan and I can tell you that Amukamara is not as good as advertized ( OSU proved it this past week ). He's a former RB turned CB who's not too flexible. He's fast and has excellent size, but he's too stiff for the position. Jenkins hasn't shown much when it comes to coverage.
I might consider him a late 1st rounder, but would prefere a 2nd round pick instead.