$40 a month and $400 of my time a month, that is the issue. At some point the lines meet and it is worth the time to save a $10000 DO bill for hight traffic I am sure. But that is a later problem.
And not just you, it's a core part of cloud computing's monetization model.
Additionally, if anything goes wrong with your Digital Ocean account, Digital Ocean will not help you. So you are running a high risk.