The thing to remember is that this does not just come down to a "per unit" cost issue on the proposed order for 75'ish aircraft....though that is obviously a key driver.
The business plan for easyJet envisages hundreds of aircraft at multiple bases across Europe. Sticking with one supplier means they will be less inclined to give the best price in future orders if they feel confident they have you as a captive customer.
Obviously a saving on these aircraft of such a huge sum is extremely significant, but the real (potential) benefit which the assessment is addressing is future orders.....aircraft 125-175, 200-250, etc. Now if easyJet are by then operating 50+ aircraft of each type, then that negotiation really would get interesting!
In trim