I would be very sceptical at £1000 per A.
Somewhere between £10 and £100 seemed to be typical among my dc's friends.
But I do find the "reward for effort only" patronising. Do you think that at their age they haven't worked out a B isn't as good as an A? Don't you think that if they're disappointed, saying "you put in so much effort, so proud" that's actually worse, because they're hearing "I didn't think you were capable"?
And "we're going out/holiday whatever because they worked so hard" is not really a reward, is it? That's for the end of exams, not results day.
Fine, go out when they've finished the exams to celebrate the hard work finishing, but having got the result then if they're disappointed, then they're not going to feel "oh well my family thinks I worked hard, so it doesn't matter", but if they have done well, then they may feel that you don't really feel that pleased because you were going out anyway, so you feel that their good results don't matter.
And rewarding for effort, how does that actually work? Can you tell?
And how do you decide if they deserve the effort reward or not? It's subjective.
One child may apparently do little work because they're quieter about it.
What if they haven't put in full effort? Are you not rewarding them?
If they got all top grades are you then going to not reward them for not putting the effort in, even though they didn't need to?
I was always told how much cleverer my siblings were and, oh, how hard they worked. You know, one year my db even decided he needed to work Christmas day... This was at the point he'd been asked to help with washing up (no dishwasher), and he didn't actually do any work, but my parents fell for it. They still hold that up as how dedicated he was.
I was told what I was good at came easy to me and I really didn't put the effort in, but they were so much cleverer than me in every other way.
I didn't actually feel I deserved my results.
It was years later when it suddenly clicked to me that on paper I had actually achieved as well/slightly better than them. So either I was cleverer than I had been told, or I'd worked harder than everyone thought...
And when I thought about it I realised that I had worked hard. I had done a lot of work, but I made much less fuss about it. All my parents would have known was I was in my room with no huffing about everyone else having to be silent or spending Sunday lunch them that I had solved Fermat's last Theorem finally after days of work.