I'd add, though, that "efficiently" isn't just whether you can build cheaper than buying. It's if the cost of building the functionality is cheaper than the pro

I'd add, though, that "efficiently" isn't just whether you can build cheaper than buying. It's if the cost of building the functionality is cheaper than the profit/growth you can generate with a sellable product built using the same amount of developer effort. That's a very, very different (and probably more expensive) proposition. That's why executives should make the decisions, not engineers! Back in the dot-com days, I worked on a project to build some functionality in-house that we could easily have bought off the shelf. The engineers argued that we'd save the company a million dollars. But frankly, we just wanted to do it because it was badass. And it turned out our solution would actually have cost us more per-system than the commercial solution we sneered at (hardware costs, not just development cost). Six man-months of engineering when everyone knew we were racing the clock before the money ran out? Absolute stupidity. If I were CEO/CTO and caught wind of such a project, I'd tell people that if they lifted a finger on it, they'd be fired. But that's a very different perspective than I had back then. Risking the very existence of what could have been a very big company in order to someday save a million dollars? Feh. (Of course, no one stopped us, because the CTO was just head nerd, and the money execs were busy fundraising rather than supervising)