I think it was an error in letting him walk, no matter how badly he's perceived to be playing in Ten. He's better than Colin Brown, no doubt. The Bills are 20 MILLION dollars under the cap. There's no excuse to let ANYONE walk when you have that much coin laying around.
I don't know if this will change your opinion at all; I just like to point out that the team won't have all that cap space forever, and decisions need to be made. They are currently paying Urbik $4M/year, and just extended Eric Wood for $6.5M/year. In another year and a half, they'll need to re-sign Cordy Glenn to LT money ($9M/year minimum). You can't expect to keep every free agent, and if you intend to be a good all-around team, paying $28M/year to 4 players on the OL isn't practical. Good teams don't do that.
You won't find a great team paying top 10 money to more than 1 or 2 offensive linemen. According to Spotrac, here's a list of teams paying top dollar to more than 1 (with average annual salary in parenthesis):
Jets - Mangold (2) and Ferguson (7)
Broncos - Clady (1) and Vasquez (10)
Panthers - Gross (6) and Kalil (1)
Rams - Long (8) and Wells (4)
Seahawks - Okung (9) and Unger (6)
TB - Nicks (1), Joseph (5), and Zuttah (10)
NO - Evans (3) and Grubbs (7)
That's only 7 teams in the 32-team league that are paying more than 1 OL top 10 money. When Wood's contract kicks in next year, he'll be the 4th-highest paid C in the league in terms of average salary, so saving the big money for Glenn, instead of Levitre, was a wise move IMO.