I suspect there was already a pay-off amount in his contract. I doubt the sevco board were completely convinced he would last the whole 3 years, and put a sacking figure in. I'd imagine that's fairly common practice now given how few managers last three or more years at any club.
I'd guess they would try and put a similar clause in with McInness - so he wouldn't even have a nest egg to fall back on after getting the sack when he inevitably fails to overtake celtic.
I'm a bit surprised this wasn't mentioned at the time, but a few weeks ago there was a discussion about pedro under-performing on sportsound involving that prick Young. He said that journalists would often be used to sound out (tap) potential managers and players, and without being specific alluded to McInnes not willing to go near the sevco job.
From that, I guess they tried to get him after Warburton was sacked, and were told there was no chance.
I think the Bristol experience scarred McInness, and that put him off Sunderland and would put him off the huns (for now anyway).
I suspect they'll go for Kenny Miller - avoids them having to pay another wage