I don't know how good or bad he will be when his contract advances.
This is just like stocks, they bet on his potential and the market for people with potential is obviously small. It's not like RBs when dudes coming outta college swinging, most young guys are below average at best. And it's not like the proven old heads are much better right now.
So as far as the numbers goes, the economics likely make sense.
Now you might have a better read on his potential than the FO:s of the NFL, that's another story