Usually there is a call-out fee and a minimum cost to diagnose, hourly rates are around $100 or more per hour.
General cost would be two hours at around $100 per hour or more.
All Acs should be serviced once a year for ducted and split as a minimum and for commercials we suggest quarterly or half-yearly.
AC coils needs to be cleaned once a year.
Yes It is mainly for commercial.
If they are installed properly they will never needed to be added.
It can be due to short of gas, blocked filters, faulty electrical components etc.
If they are blowing particles, dirty marks in the outlet, uneven air flow your ac ducts might need cleaning.
Replacing an AC Unit starts from 680 to 5000 or more depending upon your situation.