Some advice you didn't ask for:
1) If you own a home, you really should own a decent plumber's
snake. Often you can clear an obstruction yourself. Even
if you can't, you can generally determine the approximate
location of the clog.
2) Always phone the city first if there is any chance that the clog
is their responsibility. Using your snake may give you a hint
about the location.
3) Ask around. Almost every neighborhood has at least one person
who has a long list of reasonably priced handmen. The gentleman
that some of us hire for sewer issues charges about 1/2 what
Roto-rooter charges. He's got good equipment, including sewer
cameras. He's also honest - he questions folks over the phone
and advises them to call the city first whenever that seems prudent.
4) Use the phone to get several quotes. Be certain that you understand
what you will receive for a fixed price quote over the phone. For
example, Roto-rooter may charge $x.xx for a basic call, but then
an hourly rate of $y.yy gets tacked on if the service call on-site exceeds
45 minutes. If it is less than 45 minutes, you still pay the flat rate.
5) For all home repairs that you don't feel interested in doing yourself,
have your resources lined up before the emergency occurs.