The same for me as my cobbler charges $12.00 per pair of shoes (heel and toe). I purchased them in bulk about 10 years ago from a supply house and still have quite a few left. The supply house closed and I'm having trouble finding another.I'm not sure if your question is about metal guards only or heel/toe guards in general.
I buy the rubber, nylon type in bulk from my cobbler
and put them on all of my dress shoes, heels and toes.
I'd rather use these than have my heels and toes wear down.
I've never used metal guards and probably won't. I wouldn't want to sound like a tap dancer with every step or possibly slip on a smooth surface.
I can't tell you how to better market your work or product.
Just inform your customers of the benefits.
Of course, if you install them you won't be seeing their shoes for heel replacements soon.