Yes, they are mandatory. It's in the DOS regulations.
(c) Program eligibility. Sponsors designated by the Department of State to conduct an au pair exchange program shall;
[...]
(6) Require that each local organizational representative maintain a record of all personal monthly contacts (or more frequently as required) with each au pair and host family for which he or she is responsible and issues or problems discussed;
However, to my knowledge there is no rule about cost or notification. "Interest-prohibitive" most likely won't count and as the monthly contact is a requirement I wonder if the cost shouldn't be covered by the HF.
Our meetings tend to happen at LCC's place and inexpensive to free. Our agency has a 10 out of 12 rule and they won't get their completion certificate if they don't show up to 10. Also no extension if they haven't been to 10 meetings.