Here are a couple of responses I use:
"Would you ask Picasso to paint your garage door just because he was 'a painter'?"
Would you ask Chanel to hem your pants/sew on buttons just because she 'made clothing'?"
I guess I've heard this so often that I no longer have a problem saying, "you couldn't afford me!" and laughing. I definitely agree that, since I hardly do my own mending, I'm certainly not likely to do someone else's!