First I want to address the point that any contractor, or employee you have working for you, you should trust. It’s unprofessional to hold a site hostage when you already paid them to make it for you. If you don’t trust someone don’t work with them period. That being said, development is a lot of work, and non-technical people often underestimate how much work simple changes can be. If a technical person says something will cost a lot of money, you should trust them enough that they aren't trying to rip you off.