The Steelers have to weigh the options of matching that offer sheet, giving him a long-term contract or letting him leave, receiving the 91st pick in the 2013 NFL Draft as compensation.
The long-term option isn't a terrible idea, but the crux of that position rests almost entirely in what Sanders would be willing to accept.
As it is, the Steelers would add $1.177 million to their cap if they chose to match the Patriots offer sheet for a one-year deal, according to Jason at Over The Cap (an outstanding resource on the topic).
A quick and easy comparable contract is the one the Lions got Mike Thomas to sign in 2011. His contract was a five-year, $19.045 million with $9 million guaranteed deal with a $6 million signing bonus.
Thomas, a fifth-year veteran, has a bit better statistical track record and less of a history with injuries. But he's similarly damaged by a lack of touchdowns (Thomas has seven touchdowns in four seasons, Sanders has five in three). Thomas is only 5-foot-8, where Sanders can't boast that disadvantage.
Sanders averages 34 catches, 420 yards and 1.6 touchdown a year. Thomas averages 43.8 catches, 447 yards and 1.8 touchdowns a season.
While the comparison approach fits for contracts, they are all specific and unique to the player, the team, the salary cap and the year. A fair comparison to Thomas's contract may not be possible, considering two years (read: two drafts and the contracts of Mike Wallace, Vincent Jackson and Pierre Garcon have altered the market significantly) have passed since he signed it. And Thomas's career arc was well ahead of Sanders before a disastrous 2012 season in which Thomas posted career lows in most major categories.
Perhaps more comparable is the contract signed by Falcons WR Harry Douglas. He inked a four-year, $12.5 million contract this offseason with a $3 million signing bonus. Douglas's averages: 29.5 receptions, 371 yards and less than one touchdown a year (three career TDs in four seasons).
However Sanders and his representation would break it down, there are several players who have recently produced on a comparable level as him with contracts within the $3-6 million guaranteed range and total value of $12-18 million over four and five years.