Psychotherapy is seen as prohibitively expensive because of the way it is currently practiced where the patient must see the therapist on an ongoing basis. This isn't necessarily a bad thing, it's a lot like seeing a personal trainer on a regular basis but it's certainly an expensive way to stay fit. Psychedelic therapy seems to be an option that helps make a much smaller number of sessions "stick". If you only have to do 3 sessions a year then it means the therapists can see a much larger number of patients and the overall cost for each patient could go down.

My opinion is that most people could handle (small) group therapy as well which could further reduce cost and might actually have a number of benefits for those involved.