The rule of thumb says, "If you are not a consensus #1 round pick you stay in school as long as you have eligibility." The money drop off from the first round is huge and every round after that is also a big drop. It was a bad decision when he declared and a multi million dollar mistake since the end of the draft. I wish him all the best, sorry he screwed the pouch on such a big life choice.
Big time college football is like a a big springboard into life. Too get the most out of it, jump out as far onto the spring board as possible to get the most lift and spring to catapult yourself out into the NFL-life's work. To waste the potential of the springboard then short hop it and get crappy results. We just witnessed the second with typical, crappy results.
If he stayed, he would have been the #1 weapon in Urban's arsenal on an always explosive offense and would have realized a nice financial windfall in the 2017 draft. That is what the "springboard" will do for you when used to its maximum potential.