He'd bring in a first no problem.
Look at what a first-round guard would get you. A 1st round guard is going to suck for his first 1-2 years, because they almost invariably do. By year 4 or 5, you hope he's hitting his stride and becomes a Pro Bowl level player. Chris Lindstrom is your optimal scenario. Worst case scenario is like Chance Warmack. Your guard busts and never becomes any better than a fringe level player.
Martin is going to give you near guaranteed Pro Bowl level play this year. He does kind of become a year to year proposition after that, but the cliff isn't in sight yet. I'd guess he has 3 more quality years in the tank, and after year 4 or 5 he's probably off your roster.
Martin will cost more money than a rook, but he'll bring in far better results in the short and medium term than almost anyone you could pick. If you are a team with a 2-3 year horizon, like say the Jets, I'd way rather have Martin than a rookie RG. That rook probably wasn't going to play like a stud by then anyway. Martin is almost a guarantee to.
I would prefer to keep Martin on this squad, as I think he's worth more to the Cowboys than a 1st round pick at this point. You could make an argument for the famous Belichick approach, dump 'em a year too early rather than a year too late, but I don't think Martin is even two years from the cliff yet.