I do a pump out once a year. Things are not working perfectly. The local borough requirement is every 3 years, tops. I've used the same people for the past 20 years. I'm pretty sure I have a 500 gal. tank but have mostly been charged for a 1000 gal. tank. The cost had floated around $120-$150 for many years. It went to $180 last year and I just paid $360 recently. I'm sure that their costs have risen, but it seems that every good, food or service goes up 40% to 100% in each increment.
Interesting you have the 3 year requirement as well. I talk to people outside of the couple counties around here and they're baffled and never heard of the requirement.