It's great that you recognize that the last 1.5 years were the period you feel like you did things to justify that bill. However, much like juniors everywhere, you justify all of your pay because we are not paying you for your skill at that moment, but for who you will become.
Even more so for PHD work because the expectation is that after the training you will produce many things that make the cost of training you essentially negligible.
Even more so for PHD work because the expectation is that after the training you will produce many things that make the cost of training you essentially negligible.