[another-dave]

But you do this in the non-software world all the time — if you leave your car in for a service, you'll get charged £X base on it taking them Y amount of time. Some cars will take them ±10% but it balances out in the end.

Or if you're getting building work on your house, they'll caveat that it'll cost you £X, saving any unforeseen circumstances that arise (if you've asbestos in your attic, they're not going to stick to the original price and just eat the cost)

In consulting, most clients want a fixed cost for a fixed scope, with any deviations coming as change requests & billed accordingly. Generally people are hestitant to agree to an agile model unless you've already got a lot of trust built up.