Crosbys first 8y of the contract have
12, 12, 12, 10.9, 10.9, 10, 9, 9.6 = avg 10.8 per season.
In 2013/14 the cap was at 64.3m so Sids avg salary was around 16.8% of the cap. This would mean McDavids deal would be close to 100m for 8y -> 12.5 per year.
In the end, i think it will come down to this amount.
They will have issues, look whom they have to extend this and next year. They will pay ~40m for two lines.
Crosby's cap hit has been $8.7m for years, never gone above. Are you looking at salary rather than cap hit?