Ouch Joe!!!
Maybe the scope of work that he accomplished for you on your property was much larger than the scope of work that was bid for by the other plumber.
Sometimes the contractor will come in and tell you a low bid, but then once they open things up tell you you need to spend more to get it corrected because that was not in the bid. It is a tactic used to get jobs.
What I am saying is maybe your go to guy for plumbing is making a reasonable profit and not gouging you- perhaps???
Maybe they had to cut through clay soil versus loam, or tunnel under a driveway???
I watched the work digging for a sewer in the road in San Antonio and they must have been 20 feet deep- this was on a commercial feed on a main road.
On the face of it it is a huge amount.
Did you get a line item list of what was done or did he just tell you this is what you owe boss get out that checkbook.
I hope you get this worked out to a comfortable price because that's an awful big number to just jump up and bite your checkbook - that's an awful lot of Mc'Donald's cheeseburgers.
Good timber does not grow with ease, The stronger wind, the stronger trees--hang in there Joe.
Good Luck!