What’s your thought process when deciding wether to build vs. buy software (saas) for a new product?

Is it a core feature for my business?

If yes, build.
If no, buy.

How much does it save me? The cost can be money, time or efforts.

Tyler Tringas sold his incredibly successful StoreMapper and the acquisition process was incredibly simple (asset transfer), because he used all online saas products to manage and operate his business, which allowed the new owner to simple acquire accounts for different products!

You will save time, money (your time to build product is much greater in value), and stress from dealing with the issues of maintaining a supportive piece of your main product.