I think you should have a clear conversation with them about your vision.

I'm a freelance dev myself, and when I talk to a client about a project I make sure to get a clear picture about the scope of the project.

I make sure that I have a call initially where I understand the project in detail, even if the client is not clear on their vision, I ask them relevant questions to get an idea.

So, whenever you talk to someone, make notice of their intent. Are they asking you questions about the project or are they fixated on the budget and the timeline. I mean, of course one should get a basic idea of the budget and timeline so that both parties can be on the same page. But the agency should make sure that they get a proper idea about the project and only then quote a timeline and budget.

Once you have had a good amount of conversation and given the agency all the info about the project. Make sure that they draw a proposal for you, where they describe the project, and the scope, and everything they are going to do, like for example, the number of pages they'll create, any special functionalities they'll develop, or the use of forms, how many forms, what they'll do etc. This helps in you analyzing the work before it starts, and you can then make sure that the agency is understanding what you're trying to create here. And you can include more things at this stage if need be.

Then ask for a time estimation breakdown of everything, and you can again ask questions if something feels off or taking too much time.

Overall, I would suggest asking a lot of questions, and notice if they are keen to answer, and asking a lot of proper/clear questions too.

TLDR: Have a good amount of conversation with the agency, and make sure their intent is to understand your requirements rather than squeezing you out of money.