Airbrushing is a fine art and takes a lot of patience and practice to get it right.
I have the flag faded in and out over my bonnet and wing with a 3d effect white swirl into it.
Price for airbrushing can vary a lot, but you would expect to pay a hell of a lot more then £40-£60 for a bonnet. For example my bonnet and wing would be £150.
However, please take no offence here I highly doubt anyone would want that particular design on a K10 bonnet, you would be better off offering airbrushing to the customers design or come up with a list of designs.
i.e book a car in for a few days and work on it like that with what the customer wants.