The Pats are passing team in a passing league. I don't know where you can find 30 to 40 carries a game for the running backs. Not counting QB and WR carries, the Pats averaged 24 rushes per game in the 2014 regular season. Back out Ridley's carries an the other running backs averaged 18 carries a game split among them.
So lets be clear. If you pay a back like Ridley more than Gray, your benefit is potential extra yardage on 10 to 12 carries a game. On average, that is an absolute max of 10 total yards per game for whatever extra you are paying. The difference between two average NFL running backs on 10 carries per game is a very, very small difference in the overall performance of the team -- a difference that would not justify much additional cap expense in a salary cap league.
BTW, on roughly the same number of carries last year, Ridley averaged 3. 6 ypc and Gray averaged 4.6 ypc.