Anyone know the average salary of adcoms?
Schools differ I'm sure but at my school the adcom members get no salary support ... we are volunteering our time and it does take time away from our usual responsibilites (for which we are paid). Much of the review work (other than interviewing) can be done at home or after hours. We are offered coffee, tea and bottled water at our meetings... maybe a cookie or a piece of fruit (no booze

) Once a year there is a thank you dinner for the ~100 adcom members and interviewers.
However, there are many other costs associated with admissions offices:
Wages and benefits for: Dean or Director, receptionist(s), other staff members who coordinate interviews, manage application files (scanning and uploading "updates" etc), handle business matters.
The cost of recruiting students also falls on the admissions office and this can include cost of traveling to and attending med school fairs and making visits to undergrad institutions, as well as the printed materials and promotional items the school gives away. The cost of developing and maintaining web sites for promotion and recruiting.
Space. Yes, even though the university owns the building, every "cost center" has to contribute to a general fund according to square footage. This covers utilities, security, housekeeping, etc.
Furnishings and electronics: obviously office furnishing, computer systems, phone, fax, photocopier, and upkeep on these items.
Costs associated with interview day including meals/snacks for interviewees.
Costs associated with second look including food, printing, swag (tee shirts, bags, etc).
Costs associated with criminal background checks.
I do suspect that some schools charge an arm & leg in the hope that only those who are very interested will apply but it doesn't seem to work for schools in good locations that some people consider "safeties".