It sounds a big hike but in reality that might not be that much
You got this low price in the beggining when your planes were x. Now your planes are at least 5-10 times more and your revenue also in similar analogy. The rate is in relevance with how much you pay for fuel.
In order to find out if the deal is good or not, you should divide the fee with the actual fuel monthly spending in this specific base.
From this division you can estimate the percentage of the spending fee vs actual current spending
Ie in your example the fee is 5 million
If the actual monthly fuel spending (4 weeks) is let's say 500 million, the fee is some 1% of the actual spending, so your clear benefit is 8,6-1 %=7,6%
If instead the actual spending is 50 million then the percentage shows that the deal will cost you 10% and it will give you a discount of 8 something, making this a bad deal.
It is all relevant to how much you are going to really benefit and not per discount number. A critical factor also for the decision if a deal is good or not is the projection of the growth. If you plan to double in the future the planes and flights from that base , then it might be worth to choose a deal with lower currently discount but that lasts 4. Years instead of 1.
My 2 cents