Extremely low.
Sure, there's a guarantee of 20m if they make CL but that's really just Pirelli protecting themselves. Inter got bent over and fucked with that deal. 10m from a main jersey sponsorship and they won the CL less than ten years ago. That's provincial, midtable level sponsorship.
Its a fair deal considering the circumstances
A.They are in huge debt and have been a mess financially for yrs now
B.They play in Serie A.Juventus is its only marketable team and one with a bright future.As an overall product,Serie A is still poor.Lot of improvements our needed if you need more casuals to tune in watch Serie A games.
C.They havent played CL since 2011-12 season.Their brand value is diminishing every yr.
D.Barely any marketable players in the team.Pretty much a selling club now who have to rely on sales to make transfers.
Considering all these reasons,you can understand why they got such a low deal.Ofcourse Pirelli would want to protect their interests.Inter are like a no-hope team right now.