Matt Cassel’s duties last season were clearly defined. The fourth-year quarterback for the New England Patriots wasn’t supposed to do much of anything.

Cassel’s job was to ride the bench as the backup for Tom Brady, who had directed the Patriots to three Super Bowl championships since 2001. Their relative importance was reflected in their paychecks: Brady would be paid more than $14 million in salary and bonuses, Cassel just $539,140.

But Brady suffered a season-ending knee injury in the very first game of 2008, pushing Cassel into the starting role, where he did unexpectedly well. He completed 327 passes for 3,693 yards and 21 touchdowns, leading the Patriots to a record of 11 wins and five losses.

That made Cassel the most cost-effective quarterback in the National Football League, according to a new Portfolio.com/bizjournals study of pay levels and on-field performances.

Portfolio.com/bizjournals analyzed the cost-effectiveness of all 36 NFL players who threw at least 160 passes during the 2008 regular season. The highest scores went to quarterbacks, such as Cassel, who delivered big results on the field while drawing relatively small salaries.

Each quarterback’s salary cap value, defined as his base salary plus prorated bonuses, was matched against his performances in four statistical categories: completions, yards, touchdowns, and interceptions. (Salary-cap values are annually collected and reported by USA Today.)

The statistics for each player were converted to ratios, such as completions per $100,000 of salary cap value. Above-average ratios (more completions, yards, or touchdowns, and fewer interceptions) received positive scores on the cost-effectiveness scale, while below-average numbers were scored negatively.

Cassel and Tyler Thigpen of the Kansas City Chiefs emerged as the NFL’s two most cost-effective quarterbacks. They earned 6.79 and 6.00 points, respectively on the analysis. Trent Edwards of the Buffalo Bills was a distant third with 2.56 points.

Thigpen’s story was similar to Cassel’s. He, too, was a backup who was elevated to a starting job because of injuries. Chiefs starters Brodie Croyle and Damon Huard were both knocked out for the season, giving Thigpen the chance to complete 230 passes, including 18 touchdowns, while earning $375,640.

Thigpen actually edged Cassel in three of the study’s categories, but Cassel was decisively better in the fourth, winning the overall cost-effectiveness crown:

• Completions: Thigpen completed 61.2 passes for every $100,000 he was paid. Cassel was on his heels with 60.7 completions per $100,000. No other quarterback did better than Edwards’ ratio of 33.5
• Yards: Another narrow win for Thigpen, who racked up 694.3 passing yards for every $100,000. Cassel was nearly as good with 685.0 yards per $100,000, and Edwards again was third with 368.6.
• Touchdowns: No surprise here. The only two quarterbacks with more than two touchdown passes for every $100,000 were Thigpen (4.8) and Cassel (3.9). Third place went to J.T. O’Sullivan of San Francisco, with 1.6 TDs per $100,000.
• Interceptions: Indianapolis’ Peyton Manning, the top-paid quarterback, with a cap value of $18.7 million, tossed only 12 interceptions, equaling 0.06 per $100,000, the NFL’s best ratio. Thigpen was the worst with 3.2 interceptions per $100,000. Cassel (2.0) was considerably better.

Rounding out the top 10 in cost-effectiveness after Cassel, Thigpen, and Edwards were Aaron Rodgers of Green Bay, Joe Flacco of Baltimore, Ryan Fitzpatrick of Cincinnati, O’Sullivan, Jay Cutler of Denver, Jason Campbell of Washington, and Seneca Wallace of Seattle.