7th round picks have no cap hit if they don't make the team. Likewise, they can also be put on the practice squad and saved. Either way, if they are on the final roster its for the rookie minimum and if they're cut they are gone. No bonus, no prorated signing bonus. If they're injured and can't be cut, they make the rookie minimum. Ellis blows out his knee 2 days into mini-camp...he still gets paid until he's healthy.
Finanically, he's a much bigger risk than a 7th rounder. If a team wants to bring him in, they want it to be for a lower risk (IE, chepaer).
Thus, he isn't worth a 7th round pick. It's about more than talent