How much do you think a rookie at #12 costs? We're not talking about great guys who can come in and start, we're talking about depth here.
'Cept that most very successful teams have well-rounded teams (and yes, the Patriots have been well-rounded recently - they have played some very good defense to win a number of games). You don't get that way because you specifically focus on one area, you get that way because you target the best players available.
Lets take a look at how various good teams built those lines:
Patriots: 2nd rounder, 1st rounder, 5th rounder, UDFA, 3rd rounder
Colts: 4th rounder, 5th rounder, UDFA, FA, 1st rounder
Steelers: 4th rounder, 1st rounder, FA, 6th rounder, 2nd rounder
Cowboys: 2nd rounder, FA, FA, FA, 2nd rounder
Packers: 2nd rounder, 7th rounder, 2nd rounder, 3rd rounder, FA
Chargers: 2nd rounder, UDFA, 3rd rounder, FA, 6th rounder
Bills: UDFA, FA, FA, 5th rounder, FA
As you can see, there is no "right" way to build a line. What matters is that the FO evaluates players well and acquires good players, regardless of where or how they are acquired.