Main sanitary drain under house broken.
Today I noticed that while doing a load of laundry, my bathtub was backing up with dirty water and my toilet was leaking around the base. Upon googling I figured it was a main drain clog, so I called a local plumber. He initially agreed the main line needed to be unclogged and proceeded to do so. Once he got in there, he noticed there was a lot of dirt that was in the drain, and was unsure why. A little while later, and still unable to unclog the main drain from outside the house, he went to the toilet in the master bath where I had first noticed the problem. After he was snaking the toilet for awhile, I peaked outside and noticed that his "snake" had come up through the front yard garden and was down the driveway. He told me that my sanitary drain pipe underneath the house had a hole in it somewhere, but that he'd have to take a camera down the pipe to know for sure what he's dealing with. With it being a Saturday after hours, I decided that I'd wait until normal business hours to do anything else. My house was built in 2000, with PVC piping. I have several questions:
What are the causes? There are no large trees anywhere near my house so I am fairly certain that's not the problem.
Is it possible that while trying to clear the clog, he could have busted the pipe?
Most importantly, how much, on average, would this type of repair cost?