The client shapes the contract. If the contract is not fulfilled, that firm is never hired again. The next firm is likely aware of this in advance and handles the situation differently. This works best for larger companies. Random checks and price comparisons with regular prices at hardware stores quickly reveal which companies are a) terrible at negotiating b) concealing discounts. If you pay 100 SEK for a product as a private person, the company won't be able to justify the same or even a higher price in any way. This applies to everyone and if you purchase the product in an established store.
It's worse for the public sector, which has legally mandated public procurement that can be appealed despite deficiencies in the previous contract.