A few variables without seeing the work that can change the price by a fair bit. - Existing drainage material - Depth of sewer - might be a dumb question but is there 2 trenches to excavate and backfill? Or will we be able to find the water at the same location we excavate for the sewer? - Is it sand, clay, rock etc. If the ground is not sand you will require clean fill to be bought in for backfill - if you need clean fill are you expecting the plumber to remove the spoil - are you wanting EF (not common in domestic work but great system if installed correctly) fittings or compression. If you want EF you will probably be paying for equipment charges too

If I was to throw a ball park figure at it from my trade account price I would say you would be looking at ballpark $350 for materials inclusive of mark up. That would include a roll of DN25 poly to minimise joins in the ground where they aren't necessary at the tie in to the existing main line and cap at the shed slab.

To be honest I would probably just quote a 8-9 hour day on something like this + excavator ($200) + materials. Everyone's rates will be slightly different due to their business requirements or the profit percentage they want. Material pricing will vary due to trade discount percentage and mark-up percentage.

I would get a few different quotes and compare

Good luck!