I think the chance of the Pats trading Welker right now is miniscule. Besides his obvious value, Wes is a Pats-style player and has been a good soldier by signing his tender so that he can be available for minicamps. As you say, every Patriot is tradeable, and I think that if someone made a huge offer the Pats would listen. But that's very unlikely.
FWIW, I think that the Pat's aren't cheap at all. But I think they place a lot of importance on not letting ANY player dictate terms or place himself above the team. They don't respond to public tirades (Mankins), whining and selfish behavior (Moss), holdouts (Mankins, Branch) or threats (Mankins). But they DO reward loyalty. The deals that Vince Wilfork and Logan Mankins ended up getting were very equitable in the end. They were not "bargain" deals for the Pats. But the Pats didn't give those guys their deals until they had toed the line somewhat - Wilfork by signing his franchise tender, Mankins by reporting and playing well under his reduced and pro-rated RFA tender. Even Brady wasn't exempt from it, but he was rewarded very well when he showed up for work under the assumption that a long term deal would be worked out.
I'm not sure why exactly the Pats do business this way. It would seem easier to just pay the guys and be done with it. But my impression is that the team puts a lot of emphasis on showing that they can't be bullied or coerced, and that the team is greater than any player.
Now that Welker has extended an olive branch by signing his tender and reporting, I would expect the Pats to give him a deal in the range of what Randy Moss got after 2007: something like $27M/3 years with $18-20M guaranteed. 4 years is possible. I don't believe the Pats would agree to a longer contract, unless the last year is completely voidable.