This league is so open offensively now that running backs are a dime a dozen. If you're going to spend big on a RB, it needs to be one of the top ones around (AP, for example); busting on an RB is a huge sunk opportunity cost to improve the rest of the roster.
According to the article I posted over in the offseason thread, we have cap room of $28 million over 51 players. The analysis makes it seem like Martin and Incognito are both on the books, when neither will be next year. So with that money, plus the Incognito/Martin money, plus the suggestion that Mike Wallace gets a restructure, we need to find the following (draft or FA):
3-4 OL (both tackles, 1-2 guards depending on if you think D. Thomas will fill the spot opposite Nate Garner. There's also the notion that we shift Pouncey to LG and sign another center, which I'm OK with if it saves $$$)
1 TE (even if you assume Keller is gone and Clay is a legit TE1, Sims and Egnew were both kind of uninspiring.)
Resign 1 of Soliai or Starks
Fill all remaining starting holes with a warm body (Nolan Carroll or replacement, OL depth)
That's the absolute bare minimum. That doesn't say anything about addressing the dumpster fire named Wheeler, discussing running game upgrades (this slide show (#4) suggests Anthony Dixon might be a legitimate upgrade over Daniel Thomas, and I'm inclined to agree if he's up for playing the occasional fullback), or dealing with the organizational decision to keep or let walk Chris Clemons.
A lot depends on our new GM and what philosophy he instills, but I could see us tied to Brandon Albert as our big signing, followed by resigning a lot of our free agents and making small-type moves. I just don't see the need to trade/buy an expensive RB over keeping those picks and taking a shot on someone like an Andre Williams in the 2nd-3rd round area.