It is becoming fairly vogue to not pay RBs big contracts. That said, there is always a number that works. How much is that for you?
I think I top out at about 7 - 8 mil per year.
Think McCaffrey, Elliott, Saquon, etc...
EDIT (4 year contract for those who wanted a term.)
I think it's really about deciding whether your RB is elite or just near-elite. If David Johnson had continued being productive, his extension wouldn't have mattered. Same with Todd Gurley.
Like, do Alvin Kamara or Dalvin Cook worth that kind of money?
I think that you're right that $7-8 million a year on a second-ish contract is pretty fair. After a QB on a rookie deal, the second-most valuable asset on a rookie deal is an elite RB, because you're getting half of the productive life of that player for pennies on the dollar.
People think it's hard to be an RB in the NFL right now, wait until some McKinsey consultant gets through to a head coach. It's not an accident that Austin Ekeler was suddenly productive when given a chance behind Melvin Gordon.
IMO, an NFL-caliber, replacement level young RB can replace 80% or more of the production of a truly elite-level RB. How many multiples are you willing to pay to bridge that 20% gap?