This is the point. If we sell for 20m and buy an equally good or better RB for 15m, it's good business (not taking into account intangibles such as personality, team spirit, and the like). If we sell for 30m and it cost 35m to find a replacement of equal calibre, forget it.
While no one has mentioned it recently, what we paid for him in the first place is totally irrelevant.
Personally, I'd prefer that we keep the guy. He's excellent, knows the team and the setup, and has his best years in front of him.