Cassel (one "l") is far from proven and won't have the likes of Moss and Welker to throw to, much less the Pats' O-line, much less their rushing attack, in Detroit. That will be taken into account when teams (read: Detroit) weigh whether they want to spend $14M a year and a huge signing bonus on him. And most likely, they won't, regardless of what a QB picked 1st overall pick would get (if that were any consideration, the Raiders would have traded their 1st rounder overall for Matt Schaub in 2007). And the only comparison is with the 1st overall pick, since after that the contracts are WELL below what a franchise player would demand, as Cassel's agent surely will if he's franchised.
And besides, do you think that the Pats would trade for the 1st overall and have to pay that amount of money? No way.