FOSS eliminates more uncertainty than price “supply” uncertainty. Since it’s free, you don’t have to know the “demand” value it provides to make a good purchase. Furthermore, you can always add features you need.

All of this uncertainty is easy to deal with if you have a nice API. Swapping out databases, for example, is (in theory) near-zero cost. If the database vendor tells you they are charging more, it’s somewhat simple to switch (unlike, say, ad-hoc logging).