I am somewhat confused by your statement that you , Do NOT have a clean out line !
Unless a new proceedure has been created , I have never heard , of a clean out line.
Clean Out's are typically installed on ALL sewer or drain lines . The main line going out of the house is supposed to have a minimum of -1- clean out , in it .
Are you speaking of -A- Clean Out , typically a -y- fitting with a cleanout plug or line extension with a clean out plug in the pipe end ?
If so , it is NOT an inexpensive undertaking , and usually involves a backhoe , several labormen a good amount of excavating , the plumbing repair it'self , Inspections , backfill and then reseeding.
We are NOT privy to the details of the work surroundings as that information is missing, so from what you have told us , we would have to think that the costs quoted are Not unreasonable .