What is the ideal relationship between the customer and the contractor?

Question

Work organization

I often come across such stories:
1. The contractor took the order and agreed on a price,
2. The order turned out to be more difficult (pitfalls appeared)
3. The contractor had to do double (or triple) work, but the price was already discussed in paragraph 1
4. The contractor received only the previously agreed price, but did 3 times more work.
This is especially true for freelance - where everyone is pushing for the client, and so dumping the price, and then there's this. After all, in order to hook a client, you need to immediately offer a price. And if you then refuse the order, you will receive a negative review.
What ideal relationship between the customer and the contractor could be built. And according to what scheme to work?
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I will give an example "on the fingers".
The microwave is broken and won't turn on. How much will the repair cost??
Can you answer in 5 minutes????? How can you not? You probably are not competent in the assessment! I'll go to someone who will name the repair price right away!
It is impossible to find out the price of repairs without opening and checking what is broken. Maybe a fuse, maybe a magnetron. It is impossible to say the cost of repair without starting to repair.
But in freelancing, at your own peril and risk, telepathically, you need to know the cost in advance.
But the tasks are different. In some cases this is possible. But not always.
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For reinsurance it is necessary to overprice? This leads to the fact that the customer will overpay if the work is easier.
As a result, the situation is as follows - someone pays less than the real value, while others pay more than the real value. Don't you think that something is wrong here????
My question was asked because the scheme by which freelancing is currently working is very imperfect. Some underpay, others overpay

Answer 1

2. The order turned out to be more difficult (pitfalls appeared)

At this stage, you need to stop, exhale, and start talking with the customer. He's a living person. This situation can arise mainly for 2 reasons:
1. The performer underestimated his strength and made an incorrect estimate of time / cost. IMHO, his problem. With experience passes, but there are always risks. They are reinsured from them by adding a margin to the initial assessment.
2. The customer has changed the requirements. This is called a scope change and should automatically lead to a reassessment of the timing and cost (if you do not work on Agile and on an hourly basis).
Roughly, the logic is as follows:
- we specify the details of the task as much as possible in order to make an assessment
- we form and set an assessment of a specific task with specific details
- if we make a mistake in the assessment, we ourselves are to blame
- if the details of the task change (the client makes changes) - then the old estimate automatically becomes obsolete and irrelevant. Requirements are corrected - the assessment is corrected (terms and cost). This is one whole.
If the pitfalls appeared due to an error in assessing the complexity of the project by the developer himself, then the client should not pay for it. But if these pitfalls add work and can push the timeline forward, I feel it's imperative that you let the client know as soon as you know about it. Say so - some pitfalls surfaced, they could not foresee. My fault, will not affect the cost of work. It can affect the timing - so much and so much.