I'm at the beach so I'll keep this short and let the pictures do most of the talking. Assuming Brandon Marshall is the #2 WR on the Giants (measured by fantasy points per game, PPR), what is his fantasy value in a 12-team league?
About a third of all NFL teams WR2s (that is, the 2nd best WR on their NFL team) are WR3 or better in a typical fantasy league; a little over half are WR4 to WR6; and about a sixth are waiver wire fodder.
But Marshall will be 33 this year. Of the 924 WRs in the above charts, only 26 were that age. Two points: an NFL WR has to be pretty good to still be playing at 33, and particularly good to be a team's #2 wideout.
Any 33-year old who is his team's #2 is worth having on most fantasy rosters: they will all be WR6 or better. About half will be WR3 or better but almost none of them will be WR1. Most likely they will be WR3 or WR4.
Marshall of course is not guaranteed any level of production. While he probably will have a better QB with the Giants than the Jets, even that is not guaranteed. He should have less defensive attention now that he has moved from being his team's best receiver to his 2nd best but he has to learn a new system. To me, if I can draft him as a WR4 I have some upside with minimal risk. We'll see where he actually goes when drafting begins in earnest but currently he's going as a low-end WR3 in early best-ball drafts.