No, absolutely not because what he pays for it and what he charges you are two different things. Same goes for the water heater. He didn’t pay whatever he charged you for it.but I think it's my right to ask to see the invoice for the expansion tank, don't you?
If I buy something for $10, I don’t sell it for $10. Different companies will mark up their materials more than others based on their buying power with the wholesaler. Whatever flex they have is no business of yours.