Pricing of materials for craft companies

Where, please, is the limit of what is bearable and understandable in material pricing? It actually affects many trades.

Example:
Cosmo heating expansion vessel internet price with shipping costs, warranty, etc. 50 €, charged with tax 171 €.
Wilo heating pump 160 € compared to 352 €.
No, I do not expect the craftsman to spend hours researching and comparing prices. He will buy from his wholesaler and, with near certainty due to discounts, receive the components even cheaper. Sure, he possibly has personnel and travel expenses to pick up the parts.
Instead of 210, I am now paying 523 €.
Also, strange items such as small and assembly auxiliary materials make one think, since the parts naturally included seals and only a few centimeters of sealing tape were used for sealing.
As a customer, I have absolutely no chance of finding out the final prices in advance.
Recently, there was a TV report about the installation of heat pumps in houses in France, which due to the pricing there cost roughly a little more than half. This with identical components.
It is very thought-provoking.
It likely leads to more and more people procuring privately and then installing as part of "under-the-table work."
Ultimately, it also affects tenants, because if as an owner the costs of maintenance run away from me, then there is only a rent increase left.

r will buy in his wholesale and with near certainty will get the components even cheaper due to the discounting.

Wholesale prices are significantly higher than internet prices. Because someone still makes a good profit, namely the wholesaler.

As a customer, I have no chance at all beforehand to find out the final prices.

You are interested in a single final price. Namely the one that appears on the total invoice for a trade, a repair, or wherever. And you are free to obtain different offers and cost estimates. How the total amount is composed is irrelevant to you. If you do not get different offers, then that is your personal fault. Do you have a specific problem or a specific question?

What is so hard to understand that the price of a product increases more the more people are involved in the value chain? At home, the portion of spaghetti carbonara with ingredients from the discount store costs me 80 cents per person. In the restaurant of my choice, 12 euros for a smaller portion. Should I write angry comments on [Chefkoch] because of that?

Should I write angry comments on Chefkoch because of that?

Only if it was made with cream and shaped ham.

Only if it was made with cream and shaped ham.

From the bag, of course. They probably also sell it as a 1 kg package in the store.

Unfortunately, the only remedy is to obtain several offers and then carefully compare these offers. Flat rates for small parts do make sense. Do you expect every single screw and gasket to be listed? And if the craftsman then needs another small part, should he ask for your permission in advance (because it is not part of the offer)?