I know the price of each item by m2 price or tile price. I just need a formula and some filed boxes that adjust the price according to eh number inputted by the customer.
I want to avoid using an app if possible due to monthly cost and server load times
It will be a complicated request and need a lot of steps to check, if you don’t want to install the app, I recommend you to hire a professional for it. Because no one can guide you in detail without testing and debugging it for a long time.