If there is going to be a balance slider, I suggest using the NFL trade chart to determine balance for pick for pick trades.
http://www.drafttek.com/NFL-Trade-Value-Chart.aspNFL values future picks as being worth a middle of the round pick value for the round that follows the pick being offered. Example, a next year's 1st is worth a mid round 2nd. Obviously, this would make trades more realistic while adding some flexibility to move the needle either way in one team's favor.
Once this model has been built out, then you could figure out how to assign values that correspond to actual players. IMO, a punter should require no more than a 5th round pick no matter how good he is while an All-pro caliber QB should require two #1's at a minimum. Adjustments may need to be made based on the weighting value of positions in the game, but until something like this is implemented trading will continue to be a weak spot in this game.