Even though i have never charged for DMing, nor experienced a DM charging something, to be fair, 40 Dollars is a very lousy salary, if that is yoir job.
It is also a cheap price for an evening of fun for a group of 4 to 6 people.
Even more so of the DM owns all the needed materials.
Once again we are at the point where it shows that some people really don't want to pay anything for their favourite hobby.
OMG, after 3 years wotc dares to publish updated books. For the full price...
even if I have been in the position where as a student I could not afford every book. But I always knew and appreciated how cheap the hobby is.