It's being reported that it's a 5th round pick. If it's substantially more than that then I'll agree, since I think this is a positive move for the Jets only to the extent that it's extremely low-risk.
As far as what they're being paid, Marshall will have the 18th highest cap hit among WRs this year, while Decker will have the 20th. They're taking ~$16.8M in cap hits for Marshall, Decker and Kerley, while we're taking ~$13.9M in cap hits for Amendola, LaFell and Edelman. It's not an exorbitant amount of money for your at all, and it's not keeping them from spending money elsewhere. If it gets to the point where that's no longer the case, then they'll always have the option of cutting Marshall with $0 in dead money.
Don't get me wrong, I'm not a doom and gloomer here. Anyone who's posting saying that this closes the gap and that the Pats are fools for not trying to spend dollar-for-dollar with a team that sucked last year and has $40M+ in cap space just doesn't know what they're talking about. I still fully expect the Pats to run this division just like they always do, and I hated the McCoy trade for Buffalo. But that doesn't mean that the moves that the other teams make can't be sensible from time to time.