My own academic library where I work used to have a per-semester limit and charged for requests over it but we eliminated it for a number of reasons, one of which was that it was skewing our acquisition decisions. When we did charge, the cost was less than what you'd pay at a publisher's website.
It is true that some vendor content doesn't allow ILL, but it's in the minority. In 2009 the Information Delivery Services Project found around 15% of publishers do not permit ILL at all while 46% permit it with no restrictions. The rest are in-between, applying restrictions like no lending outside of the US or to commercial entities. https://idsproject.org/documents/IDS%20Project%20licensing.p...