As far as i know, Puma offers lots of money face to Nike but face to the Americans ( or how Adidas does ), does not offer a % of the sales. So even if the Nike contract looked small with 8m quid a year, there was a fat but not fixed bonus around each year.
Now I think they have a fixed 30m + max. 4m in bonus/year which is probably linked to the performances in PL and CL rather than the sales while at Nike afaik the clubs would get ~10% of the sale price of any product. Barcelona has 27m quid with Nike + sales, Arsenal had 8m + sales. I have no idea how much they would earn/year from Nike from that % but IMO it's alot, but it was absolutely clear a renewing would have been bigger than 8/year for another 5 years, their problem was to know for sure they cover their debts, so they rather chose security over a possible better affair but what they could not be sure of.
I wonder if Puma will insist now to come with as many products as Nike came for them, since they're the most important team they signed, Puma stores are rarer though and I think it's impossible to match the market share of sporting goods they had before. I will check our local stores since I'm curious, if they get anything with Arsenal till September, usually the Puma stores here had no football stuff at all.