“How much?” is the wrong first question.
While it may sometimes be the simplest way to think about options, asking how much it will cost isn’t necessarily the best way to start thinking about designers.
Design isn’t an object purchase. It’s not a finished fixed product. It’s a personalized service that’s catered to your particular needs and requirements.
When you hire a designer you’re taking on a partner to hold your hand through the programming, walk you through generating schemes, advise you on material selection, create your working drawings, and to represent your interests on site as your house is getting built. You have to know who they are, how they work, and what they stand for.