We just had our sewer line replaced. The thing is that with older sewer lines they were made with clay pipes that had joints every 3 feet that were cemented together. Over time that cement degrades and the trees go for that, ahem, nutrient-rich liquid. It is especially bad this year since it has been so dry. Sad thing for us is that the roots are from a tree we lost in the big snowstorm in October...but we learned that the roots live on and cause problems for up to 10 years! Ugh.
So, the root killer chemical that you flush down...we did it twice a year when we changed the clocks as kind of a maintenance stop-gap to hobble along, but even with that maintenance we still had to have the pipe cleaned out just about once a year (always backed up on a holiday weekend of course - Murphy's law!)
So, hopefully the flush-down stuff will help you hobble along until you can put the $$$ away to do a replacement. I hate to tell you, but the replacement is inevitable...the cracks in teh pipe will only get bigger over time and the tree roots just keep coming. And like I said, I thought we were golden after that tree came down, but no. The new PVC pipes cannot be penetrated by the roots, so after the replacement you should be good to go for the rest of your life (at least that's what we were told...hope it is true!)
Your quote is higher than what we paid, which was about $7,000...but you may have a longer run than we do. You may also have different requirements for the pipe and fittings and different permit fees.
I wish I could paint a rosier picture for you!