I have used this type of response in the past...
We cost more because we have the part in stock now, we stand behind the parts we sell, and we insure you get a quality part and the correct part for your device. But if you would like to purchase the part yourself, we will be happy to install it for you, just one less part we have to stand by. But also please be advised, if we install a part that you have provided and and it does not work, you will still be responsible for the labor charges.
Then laugh on the inside when they walk in with the wrong part.
But if you get that vibe that they are someone who can afford to pay your price, and they are just being a cheapskate, then tell them you only install parts you sell because you don't want to risk a customer getting upset at you if a 3rd party part fails.
Sometimes I will even give them just enough info that they attempt the repair themselves just to get them out of my store so that they stop wasting my time whining about prices. They act like you are trying to screw them, but in the long run they screw themselves with a botched self repair.
Had one a few months ago, $50 laptop job, he didn't like the price after looking at parts on ebay. He attempted himself. Got his screws mixed up, had one screw that "wouldn't go in all the way" so he got the bright idea to drill it deeper. He drilled straight through the motherboard, through the bottom of the laptop case, an into his kitchen table (his wife was real happy about that). Ended up selling him a refurb laptop. Made more money than what the original job would have been.