Tie rod ASSEMBLYs with the tie rods ,from the dealer would be $130 .A box parts store $75.Ive worked for a mechanic shop doing the books and running the front desk for 6 months ,they basically double the $ of the part then charge labor at a rate per hr / from $75 to $100 is not out of the range.They can do a search for time on a auto job using Alldata,(a source for mechanics to look up repair procedures and hrs to do the job).If you ask next time what the Alldata gives for hrs and if you can pick up the parts,this may save you some cash .Their is a catch ,they wont warranty the part ,most parts houses will warranty things like alternators ,batteries to a certain point but not tie rods.
Mechanic shops can be a great business to get into ,you just really have to watch your overhead,(tools), and keep a good reputation in being honest with every customer.Good luck to you and your MB.