As with any player's potential trade value, there's perceived value, supply and demand.
As far as perceived value goes, he's generally viewed right now as about the 7th - 10th RB, somewhere around there, and you may even have a leaguemate that values him a bit higher. If you are looking for a RB in return, it's a matter of contacting owners who have RBs in that range that you'd rather have.
As for placing a value on him vis-a-vis WRs or QBs, the supply and demand factor kicks in, and that depends on league size, roster size, and starting requirements. You have to know how scarce RBs are in your league in general, and on individual rosters specifically, before you can determine whether he's worth a top-tier WR or a Peyton Manning to someone.
Good luck.