There are many things that go into something like this. 10 knot tailwind is pretty standard and is usually part of the basic cert package. If operator X comes in and says, 'you know, 10 knot tailwind is really limiting us at xxx airport, could we get 15 knots?', Airbus (or Boeing) does a quick study to see if:
A) it's doable, and if the answer is yes
B) how much will it cost to certify.
They go back to the operator and say, 'yes, we can do that, but it'll cost you $$$'. Operator says, 'OK, let's do it' , contracts are signed, money changes hands, and before long the operator has a 15 knot tailwind capability.
Later, another operator hears that 15 knot tailwind is available and asks about it. Now Airbus isn't likely to give away to the second operator what they charged the first operator $$$ for, so they also charge the second operator $$$ as well.
Now, perhaps Airbus figures that once they have the option available other operators will purchase it, so they'll discount the price to the original operator, but that has obvious risks if no one else buys it.
Multiply that by hundreds of potential changes/options, and that's what the manufacturers regularly deal with...
(how expensive a single demo for a particular higher certification be ? )
You'd be amazed. I've been on fight tests where we had to fly 4,000 miles to do a remote test just to get to where we could obtain the needed conditions (and then the weather had changed and we had to hang around a few days to get the needed conditions - about 50 people total since we needed ground crew, flight crew, test crew, and cert people). And of course, if you need to add instrumentation for the testing, that adds up really fast.