Help me understand the huge difference in sewer pipe estimates?

Your post is very confusing. You say that company A will not give you a written estimate. Then you refer to the "bid" that company A provided. First off, an estimate is an approximate valuation of the cost of an item, it is NOT A BID. A bid is a binding statement offering to perform specific work for a specific price, usually accompanied by a detailed description of the scope (what the company is proposing to do), the terms (payment, insurance, whether they will pull a permit, warranty, dispute resolution, and often many other items), and a time frame within which they will do the work. Bids are always in writing, as a verbal "bid" is essentially impossible to interpret.

You cannot compare two bids unless they both refer to the same scope of work. You need to write down the scope of work so any company wishing to perform the work will understand exactly what you want done, and any specific conditions you want them to meet. Specific conditions may include timeframe to perform the work, insurance they will be required to carry, who will pull the permit, when you will allow them to work, and any other conditions you wish to impose. If the companies you invite to bid do not like your conditions, they may elect to pass on the project, or they may suggest changes to the conditions, which of course you are free to accept or reject.

In the end, you would ideally like three or so written bids from reputable companies, in the form you require. You also need to specify the type of contract you want, this might include fixed price (all risk is on contractor), hourly rate (not recommended), hourly rate with a not to exceed price, cost plus profit, or one of several other types of contracts. If you are uncomfortable preparing a contract, you may want to consult with a paralegal firm, they can help you put together a simple contract that protects you from a wide range of problems that can develop on home improvement projects.

Daniel: Sorry, I wasn't really sure the right terminology. I can try to explain a little clearer.

The guy from Company A presented me a handwritten description of the work to be performed along with a price. I wanted to compare numbers, so I asked if I could make a photocopy. That's when he gave me the spiel about "proprietary information". He wrote the total price on the back of his business card, and wrote only "ALL PIPING" (meaning the sewer pipe and the ones that need vents). This guy was very pushy, almost like he was a car salesman trying to get me to commit. Stuff like "Well, I really need to know soon so we can try to get you on the schedule...", stuff like that. He actually gave me two "options" - one to do the sewer main *only* for $5,389, and one to do the sewer main and vent piping for $10,767.

So from Company A, I have, written on the back of a business card:
STANDARD
ALL PIPING
$10,767.00

ECONOMY
SEWER MAIN ONLY
$5,389.00

Now, here's what I got from Company B. They emailed me a document listing all the details:

We are pleased to submit this quotation to perform plumbing work at your home.
This estimate is for a complete installation including all labor and material required to
perform the following work:

1. Excavate building sewer 10 feet outside the foundation wall and replace 4” cast iron
soil pipe along with the 4x3 and 4x2 wyes in the house;
2. Replumb the drain pipe to the shower so that it will be vented;
3. Replumb the laundry drain piping so that it will be vented. Provide trap primer for
laundry pan trap;
4. Acquire all permits and pay all fees as required by the Town of Billerica Plumbing
Inspector.
The cost to perform the above described work will be $3,976.00.

Note: Patching of walls and ceiling that will need to be opened is to be done by others
under a separate contract.

Thank you for the opportunity to submit this quotation. Work will be scheduled upon
return of this signed contract. Please feel free to call if you have any questions or would
like further information.

Thanks for the advice. Hopefully I will hear from company "C" tomorrow. I don't want to wait too long because my sewer main is in real bad shape, it's pitched back into the basement and there is a slow leak of water. So time is of the essence.

I know prices vary a lot by region, company, work, etc. but I was just wondering if one of the bids seems ridiculously low or ridiculously high. The only other thing I forgot to mention on Company A's $10,767 number is that it also came with a 3-year warranty. But that still seems like a BIG price difference to me.

Another question maybe some of you plumbers could clarify for me - Company B's estimate is replacing my current cast iron sewer pipe with another 10' cast iron pipe. Company C, who I'm still waiting to hear back from, says they would most likely do it with PVC. Is there any good reason for one over the other? As far as I can tell, the 10 feet of cast iron is original from 1968. After the 10 feet, it connects to a newer PVC line. So would there be some advantage to do 10 feet of cast iron from the house then connect to the PVC as opposed to run all PVC?

Thanks for your patience as I know next to nothing about plumbing.

Here is a picture of my mess of a sewer pipe.

You can see my strategically placed buckets that catch the occasional leakage. The worst leakage is the joint between the PVC and cast iron sections. There is a pretty significant downward angle leaning back into the basement.

So the house was originally on septic, that's why the sewer line exits the house horizontally instead of vertically. In the mid '90s the previous owners switched to town sewer. The cast iron section that goes into the wall continues for about 10 feet where it meets the newer PVC that connects to the town sewer.

What do they need to do, just make sure the sewer pipe has the correct grade leaving the house? I'm assuming a cast iron pipe doesn't "bend" over the years...