i got an idea, not the kid yet. i will buy him/her everything they ask, but....
one day of chores is £1. you want a bike worth £150, then you need to do the adequate chores 150 days. not in a row, your choice. £20, do the 20 days. and so on. you can have anything you want as long as you do the effort.
Now this is something different to try with the kids - thanks!