Insulation and filling the cracks in the concrete wouldn't be too bad--a bit more if the garage isn't currently finished (and you want it to be). Adding the half bath will be a lot more, and will likely require much more substantial permitting. (For instance, there may be parking requirements in your neighborhood that would need to be satisfied in some other way, and if the garage is close to the property line, it may not be allowed at all since you'd be turning it into living space.) How far is the garage from the existing waste lines? That will probably be a bigger factor than the plumbing lines themselves, especially if it's under slab. No idea what LA prices are like these days but in the Bay Area, you could possibly do a project like that for $40-50K on the low end. Whether that's worth it to invest in a rental would depend on how many years your lease term is (remember that just getting the contractor lined up and the permits approved will take months, so you're a long time out from getting to use the space--I'd want at least a five-year lease agreement for it to be worth the upfront cost) and what your relationship with your landlord is like. (They'd need to be pretty enthusiastic about the project since at the end of the day it will be in their name, and will also increase their base year value for their property taxes.)

I rented an Airbnb in LA that was a garage conversion. They had done the bare minimum: left the garage doors, put a curtain between the bedroom and garage area that was leftover and used for storage, added a bathroom and heated with space heaters. It wasn't too bad. I think they even left the sloping floor or at least I didn't notice it much. You could probably do that (without the full bath) for $20k. A bath could double that.

I’m not understanding your post - this is a property you own but rent out? Or, this is a property you are renting from the owner? If the latter, explain please how you are allowed to make permanent changes to a property you do not own.

Kitchen sink plumbing is useless for a bathroom. Sink is 1 1/2", toilet is 3" going into 4". Pool plumbing is a completely isolated system. Bathrooms require their own electrical circuit, and an old garage probably has 1. So new plumbing and electrical homeruns from house.

I think OP is proposing to pay for the landlord to do this work, not to do it themselves. (But yes, that's an important distinction--the landlord would need to hire the contractor, so you may not have complete control over the cost as far as who they choose.) I think this really comes down to how good your relationship with your landlord is and how motivated they are to improve the property in this way. If I were a landlord already interested in converting my garage to an ADU at some point, I'd jump at the opportunity to have a tenant pay for some of those improvements. But if I didn't otherwise want to make any changes, or if I lived in a neighborhood where a home with a garage commanded a premium that I'd be losing, I'd be pretty reluctant. I assume OP has already broached this with the landlord and knows they are potentially interested and that's why they're asking about costs. Either way, though, I can't imagine this penciling out for OP vs. just renting a larger home, unless there really aren't any larger homes in the area that could provide that space.

If it's just an office space you are looking for, probably about $2K-$5K could get you that space (window a/c, 2 or 3 finished walls [1 dividing the garage in half], ceiling, rug, no bathroom though, door or not depending what's there. You would avoid permitting and could take the rug and window unit with you when you leave.

First, make completely sure your landlord is ok with you converting a very valuable 2 car garage in Miracle Mile to space not dedicated to parking. Los Angeles has various requirements for off-street parking spaces (not sure about that specific area, so check the details) and regardless, that is definitely a neighborhood where most high-end renters would demand secure parking. I'd abandon the plan to put in any bathroom where it doesn't exist in a property you don't own - that's an expensive and painful prospect with negative returns.