Respectfully, when you hire a WC you are contracting that individual for the services they provide. You need to know what those services are and be fine with them providing them. It's a package deal. Once they begin working for you they will do what you have paid them to do.
You may well be invoiced for a package - the total you are being charged. It may well not be broken down into its component parts. So you may not know what price you actually paid for each part of the service they provided. But you agreed to a fee upfront so, as long as:
1) what you end up paying is what you agreed to pay
2) you got what you expected to get
there should not be an issue.
My concern with the initial posting was the apparent assumption going into it that a service provider will plan to take advantage of the client and the situation ("it would be harder for the WC to put a percentage for themselves on top of that price. In other words, us not getting a price from the venue ourselves gives the WC more power to charge us whatever they think they can get away with")
Professionals do not take advantage of their clients and there is a significant value-add when engaging with someone who is very good at what they do - it is a win-win for ALL. This is where the trust comes in - it has to be there or there is no working relationship.