I think it makes more sense for Philly to try and keep foles out of the deal to get Mariota, and to try and get some picks back by trading him to a QB desperate team, like say buffalo.
But if Philly is desperate for Mariota, Foles plays a key role. It allows them to trade with QB needy teams.
For instance if the Jets don't get excrement in free agency at QB (Fitz doesn't get cut, and all the scrubs get wildly overpaid ALA McCown).
That would put great pressure from the fans and media for the Jets to draft a QB. And with guys like Idzik getting fired for such folleys (pressure of signing a big corner and failing to do so) it's something to think about.
But if the QB desperate Jets are able to acquire whatever bounty of picks, plus an average to above average stop-gap QB. It works.
Of course this really depends on how the Mariota projections go over the coming weeks, and how QB free agency plays out.
But I think Foles could offer Philly more value by holding onto him. Especially since without him they're basically fucked if they can't get Mariota.