Here's some fun early Monero history - in 2014 / 2015 there weren't a lot of donations to cover Monero costs, so core contributors would pay for them out of pocket. We tracked all of this on a spreadsheet (that we still have) because we figured one day we'd be able to get it back from the dev fund - although we never ended up doing that😅
In total I spent well over $100k on Monero development until the FFS and donations made it more self-sufficient towards the end of 2015.
What did this pay for?
It included things like making Monero work on Windows (via msys2 / mingw), building out the entire FFS (which eventually became the CCS), all the work on the first GUI, lots of early cryptography work, all the initial embedded DB work (Monero loaded the entire blockchain into RAM until the beginning of 2015!), the work on MRL-0001 / MRL-0004 / MRL-0005, and Monero's 0MQ integration.
Monero is only where it is today because of the sacrifices and early financial contributions of people like othe, NoodleDoodle, as well as the tireless (and unpaid!) efforts of people like smooth, David Latapie, moneromooo, WarpTangent, luigi1111, binaryFate, and countless others I've failed to mention.