No matter the quotes you get and post here, Houzzers will tell you they're too high.

@Aglitter my house was built in 1941, so I'm pretty positive it is not a post-tensioned slab. Since it is an old house, the ceiling is barely 7 feet tall, so that is why I'm planning on running the DWV under the slab. What plumbing are you referring to that doesn't need to go under the floor? I'm aware of permits and inspections needed.

@TimBo We looked into a toilet waste system that drained through the wall. Reviews on it weren't that great as far as how well it worked. We were wanting to move the location of the toilet and ended up keeping it put because it was so complicated. This system comes with a special toilet flushing system. If you're really interested, let me know, and I can look back through my notes and see if I can locate the name. Seems that there were a couple of companies that offered this, again, with varying degrees of reports of the success of the functionality. Post-tensioned slabs began to be used in the 1930s. If you're sure of what you have, proceed. If not, be very, very careful.

Depending on the pump you get, a sewage ejector pump will cost you between $300 and $800.

The entire bathroom in the basement will run you approximately anywhere between $8-12k depending on the size and what you pick.

The vent must be through (usually in new construction) or on the outside of the house (usually in older homes with plaster) because it must be above the roofline whichever is easier to be code compliant unless there are certain restrictions.

In most cases, if there is a garage without another story above, we would bring the pipe into the garage and out through the roof... and if there is another story above try to find the best way to get into the attic above, most cases we try to bring the pipe through a closet wall.

My house was built in 1940, and we did a basement bathroom conversion from a single toilet and washtub drain to a full bath, utility sink and washer.

You should first look at your main vertical soil stack and also at the point where your water supply line enters the house, because this will give you an idea of the path for the existing drain pipe (main soil drain usually exits the house on the same path as the incoming water supply). Assuming the drain under the floor is on a line between the soil stack and the incoming water line, how far will your new bathroom be from that line? You must plan to demo the entire floor under the new bathroom, and enough of the cement floor to reach the existing drain pipe.

Now determine where you are going to run your 2" vent stack for the new bathroom. It must go at least to the attic, where it could join with a larger main stack to penetrate the roof, or it will be a dedicated run to exit the roof.

Once you get that figured out, you should lay out the new bathroom and figure out how all the vent lines and drains will tie together. The drain "flow" must be toward the existing drain, including the required slope and use of proper fittings where pipes connect. I have found that an experienced plumber can lay this out quite quickly, and its a lot faster than trying to read through how-to books and code compliance info to figure it out yourself.

Also consider where windows will be, if you need to install a vent fan and/or heater, etc. Look at your existing electric wiring and breakers to determine what you need anything new for this area. Adding a few LED lights is a minimal load, but an electric bath heater and the mandatory 20 amp GFCI will require new wiring and breakers.

From there you need to make your project plan and start finding contractors, or just find a general contractor to do it all. You could certainly do the floor demo yourself, but then you have plumbing to install, cement to pour, framing, wiring, etc. You will also have to comply with local code for insulation, water management, etc., but its usually not that complicated.

Bruce

Thanks everyone for your detailed responses. For the record, I’ve done more than enough research, and have plans for the water supply, the insulation, the electrical, windows, materials, floor leveling compound, city code, exhaust fan, venting rules, I’ve got it all in my plans.
I’m specifically curious on the COSTS for the DWV system, and ONLY those costs. If you have experience hiring a plumber to install DWV under a slab, please share. I could do it all myself for the cost of materials, probably for a grand or less. But I’d rather not own the liability with the drainage under my house. That is my concern and my curiosity, DWV plumber costs.

All you need to do is call a few licensed plumbers in the area and they will give you approximate cost contingent on plan approval.

The approximate cost I gave you that should include a vent installation... That being said, you not gonna get what you looking for, prices change from region to region and different contractors charge different prices.

Like I told you before your best bet is to get a few local prices.

Good luck