You should have a contract and detailed site specification done and signed by both the client and the designer/developer before any work is done.

You need to know what you are paying for and what is going to be done. There should be no hidden surprises.

A contract will protect you as you can enforce it. It also protects the designer/developer also from scope creep. Scope creep is when you plan for something at X price and pretty soon the client wants more and more.

An example would be a client I have. It was planned to be a 2 page web site. Soon the client was asking to have a page for each item. I went and figured out the additional cost, and the client decided against it.

Another example was a small 5 page site. The client had just a few items for sale. Not a problem. Well those items were actually categories. The project should have been scoped at $3,000 versus $500.