I got a esimate today from a plumbing company to replace the sewer line from the house that goess 100 feet out to the ally. This seems like a pretty inflated price to me. I estimated I can do the job myself for about $200 and plan to do just that. Does this seem like a lot to you.

To me, that price seems to be low. In my area, you would pay at least twice as much.

Sewer pipe replacement is a heavy-manual, unskilled-labor-intense, kind of work. Once the trench is excavated, actual plumbing work takes couple of hours. ABS pipe and fittings for 100' of 4' run will cost you not more than $300.00.

I don't know how you figured $200.00 for the whole job - but - sure, you can do it yourself...

Milo's correct. Your labor cost will depend upon how deep they have to dig the trench. This is determined by the frost line in your area and the type of soil that you have to dig through.. In the Tampa Bay Area we only have to go down a few feet in sandy soil. Easy digging!
Back in Wisconsin we had to go down six feet through limestone. Big job!
When it comes to trenching for sewer and water there is no, "one price fits all". If you think you're getting ripped off then get three estimates and compare.
If you can do your own work then more power to you. Good luck, Tom

I mean you can do it yourself, if you own a backhoe or a professional trenching machine, or have days to dig with a shovel and pix.

Also you have to have permits, have it inspected for proper drop and other issues.
But no around atlanta it would cost a lot more.

Sounds like you are in a city so check with the building department. They will probably say you can not do this job yourself and need to hire a licensed, insured pro. 110' line in clay 4' down ran me $4500 in Illinois