While building a data platform, how do you approach the build vs. buy problem and quantify the tradeoffs?

The build vs buy purely depend up on the business needs. If the problem faced is common and the solution is readily available which can solve the purpose then there is no need to build in-house as building takes up a lot of resources for both developing and maintaining. Whereas, if the the problem is unique and or need modifications to match the use-case, the build strategy is more preferred. You can refer my go-to guide (https://cdn.windwardstudios.com/Website/WhitePapers/Build-vs-Buy.pdf) that helps make decision whether to buy vs build.