Sadly, this is an area, like well pump replacement, where some contractors see an opportunity to really screw people over. I know of cases in my area where a contractor has six hours of backhoe time at $7-800 total cost, a few hundred in labor and supplies, and walks away at the end of the day with a check for $5,000. Personally, I would hire the dirt work out and DIY the repair, but that's not everybody's cup of tea. In larger construction work, companies avoid getting screwed in dealing with unknown situations like this is to agree to do it as a "time and material" job. This means that all parties involved are billing by the hour and do not have the chance to gouge, assuming that there is competent supervision.
In dealing with this, I would find a reputable plumbing contractor and get a bid for repairing the line, on a time and material basis, once it is exposed. Then find a small excavating contractor to do the dirt work. This should be done on a pre- arranged hourly fee basis. Have the line opened up, call the plumber in, get an inspection if required, then fill the excavation, and regrade. Make it clear that everybody will be paid in CASH as soon as the job is done. Be there the entire time to ensure that there job is getting done. There are many cases where this can turn a $5000 crisis into a $1500 repair. You don't have to be an expert to watch and count $20 bills when it's done, but it prevents a contractor from bending you over in a crisis.
