Thanks for posting!! Congrats on your interior designing gig!
I find that when calculating prices, it’s easiest to make a spreadsheet of everything you will need to buy for the room and the approximate costs of each item. This helps the client visualize the expenses instead of just seeing a lump sum, which can seem daunting. Also, since you’re calculating with approximate costs, there is always the possibility that you will spend less if you find good deals or thrifted items!
It’s important to value your time, as well. Whether you charge an overall fee, or an hourly rate, make sure to charge for your time!