When you validate your product at different price points through your marketing channels, you already know your product will sell. You can then build up your product knowing cost for advertising, and better understand your profit margins.
If you don't know your future CPA, and you start building your product, you are basically hoping the CPA will be low enough so that you can make money.
It's also a good idea to build the cheapest and quickest MVP to see whether the product will sell with renderings and 3D printed prototypes.
Do this and you can save a bunch of money and time.
My mistake was spending too much money on an MVP (Laptop stand) then realizing it would not sell after testing it at a particular sales channel (Kickstarter) at a price that allows me to profit. Now I have to scrap this project after spending all this time and money on the MVP.