When it comes to trying to come up with Welker's value to the team, to me it is more about durability, than ability or productivity.
We saw (albeit briefly) last year that Welker's production could be replaced near equivalently by Edelman, who early in the year was seeing more snaps than Welker. The problem was, Julian was injured and unavailable just as the season was beginning to crank into high gear.
The problem is durability. Welker has shown a near super-human ability to take whatever punishment a Defense deals out to him, and pop back up and keep going. Outside of a freak accident that occurred because of the turf at Houston, he has never really been injured.
Where as Edelman has been more prone to injury, despite far less wear and tear. Same could be said of another free agent who's skills and production are similar to Welker...a healthy Danny Amendola would be more than capable of filling Welker's shoes... problem is, he can't stay healthy.
Of course, for what they would have to sign Welker for, the Patriots could probably bring back Julian and sign Amendola as well. Or, if you want to think purely about the team FAs... they could bring back Edelman and Danny Woodhead for what Welker would cost them... and they could probably sign one of the following FAs to boot:
Josh Cribbs, Cleveland Browns
Of course adding to the complexity of the situation is Lloyd's bonus, which is due a mere 4 days after FA starts. If the Patriots truly want to move on from Lloyd (who proved durable and productive) and save themselves that 3 million dollar bonus he is due on March 16th, they really need to secure at least TWO of the WRs listed above (Welker, Edelman, and/or any of the others) or risk having no one on the roster at the position at all (at least no one with any real experience or reliability).
To put a lot of money into Welker is a huge risk, how long can he continue to take the poundings he has and remain healthy? The other option(s) are equally risky, to put money into alternative players who have proven to be far more prone to injury in the past.