While the remote server may be part of the cost, its also the case that independent developers need to be able to make a living somehow
Many plugins are developed by (people working for) institutions or Moodle partners and are released, free, to the community. Others are made by independent developers, or people like myself in our free time, rather than as part of salaried employment. Sometimes those independent developers will feel able to release those plugins free, other times the development and ongoing support involved in a complex set of plugins (such as Poodl) means that the time and effort involved requires that the developer needs to make a charge. I think in this specific instance, the charge has come around as Poodl has grown, both in complexity and in usage, vastly increasing the development and support commitments involved - although Justin would be able to correct that impression if Im wrong.
I totally understand the fact that individual teachers, schools and other institutions sometimes find themselves in a position where they see a paid for plugin but don't want to/can't pay for it. The frustration below is not intended as a comment about those who CAN'T pay, but is something I've run up against more than once.
What frustrates me though is the times I have seen senior managers turn around and say something along the lines of 'Moodle is supposed to be free, we're not paying for plugin xyz', but then they go out and buy a similar functionality from an entirely separate commercial product, that wont - or work has to be done to enable it to - speak to Moodle! "Moodle is free, therefore we wont pay for plugins, but XYZ - well that's commercial anyway so we'll pay way more for it than we would have spent for the Moodle plugin"!
Richard