How do You Adhere to a Budget?

I just finished my first-ever design that I am actually happy with, but there is one massive problem with it (well, 2, but the cartoonishly underpowered engines somehow don't hinder it): it is almost 90% overbudget.

My goal was a ~200k starter craft, yet the design, dubbed the "Sourdough" because of how sour I am at how much dough she costs, clocks in at 377,807 materials.
She is incredibly good, capable of defeating any DWG enemy except the Crossbones, Barracuda, Buckaneer and the godly airships, yet it is not even in the class I wanted. I even contemplated removing one of her secondary/CIWS guns and truncating the whole entire hull.
My design process was first making the primary cannon in isolation, then planning out which systems I would need, determining what size hull would be needed to fit that + armour, and building everything inside.

How do I plan ahead to prevent this from happening in the first place? How do I make sure a design I make meets a certain cost criterion?
I'd like more general advice if possible, but if all you can provide are based on design specifics, here is the ship in question.