OP
puffy
Why bother having the customer set a budget then, as if that's magically going to make the spark do the job at a cheaper price, if as you've admitted the quote could come back over or under budget. The second most important thing I think is being overlooked is most sparks won't quote, they'll only estimate. So what happens if the subby estimates within budget, but finds half way through the job that it's going to cost another £1000 because something unexpected came up?
I think you've created in your mind a simple solution for a problem that is anything but simple.
True. Do you have an experience you'd like to share? Have you ever told a customer "hey James, I need to replace x because it's out dated so it will cost an extra x"?
This is why I take onboard your feedback strongly so I know and think this properly.