I go along with housemum that saving towards a particular goal is good message to send.
I think an "earning" system is good if you're organised enough to keep it going for long enough that they understand it.
My fear is that the complexity can get out of hand. There's just so many distinct behaviours we want, like homework, being polite, sharing with brother, saying sorry when bad, eating, getting ready for school at a reasonable speed etc, etc.
To me the amount is driven largely by what they do with it. Currently if given 5 quid cash, 5yo DS1 would just buy a lot of sweets.
I guess we'll lurch towards a level where they can buy a packet of sweets and a comic. Currently we never buy sweets, and I have right of veto over comics. Sadly DS1's choice is typically driven by the toy on the front, not the content. But in general I approve of comics, a lot.
Of course, they may develop an interest in something I approve of, like wanting to read all the books by some author (even Rowling), or some hobby.
What would be good is some variant of Amazon, where I could stream a small regular flow of money, and the kids could choose books or toys. It would be pathetically easy for their system to send me a mail to confirm any choices, so that I can stop them buying knives etc.
Saving up for something in such a system would be very easy, and could be made very attractive.
Also top-ups from grandparents etc who want to buy something, but are wary of getting it right with a kid 1/20th of their age. When a kid uses given credit on the system, it pops up an electronic thankyou card, in which they can write a message, and relative gets an email saying "thank you for helping me buy the Big Book of Wolves".
Not exactly without potential problems of course, DS1 wants to read the medical book "that tells you all the ways you can die", so he can keep himself well.
Any authors in MN land want to write a child safety book with attitude ?