I did some Google searches but could not find any clear answer. How much does PDI/kettle costs for commercial usage? Is it potentially free? Can it be hosted in Azure?
It depends on your work amount. If you are looking for a very limited amount of usage, then Pentaho Community Edition would be better, since its free. But for Pentaho Enterprise Edition, you need to pay for the licence and stuff. Do not know much on the exact pricing and stuff.
But since you are looking only for Kettle, i would suggest to go with CE edition . Hope it helps :)