“buy vs. build. . . the calculus changed”

Be careful with making decisions about your livelihood based on a rational calculus. As you correctly point out, there is a threshold for which a programmer or company should not even blink at the cost of software. It's often the case that if the software they're buying saves one single hour of productivity, it's value-positive... and yet they won't buy it. Individual devs are notorious for refusing to pay a cent out of their own wallet, turning up their noses at anything that isn't offered open source and completely free. Enterprises manage to saddle what should be a no-brainer trivial expense into dozens of hours of bureacracy that cost two orders of magnitude more than the expense the bureacracy is for.

Your customers are more irrational than you are, and your appeal to them will likely need to resonate with them on an emotional level rather than logical one. I would argue that marketing is the hardest part of enterpreneurship, by far.