Shampoos and shower gels are scams imo. I order a 10 litre container of the cheap pink soap you get when using public restrooms. It cleans just as well and is significantly cheaper. I use an empty bottle with a pump on top and refill it with the container every other month or so.
Edit: If you got sensitive skin that needs special care then this isn't for you, though.
Even then, a basic soap like Dr Bronners or your favorite "hand soap" does well for most of your body. People ask my wife how her hair is so amazing, she just washes it every couple days instead of every day and sometimes has some light argan oil or something to reduce frizz. Washing hair (especially long hair) every day damages it no matter what you're doing. The most important thing is to scrub your scalp really good.
Just buy blocks of basic hard soap. Better for your skin and your plumbing. I don't know if it's cheaper compared to your 10L bottle, but it's definitely cheaper compared to normal liquid soap bottles.
If you don't have hard water, maybe. Not here. Soap scum on the tub, soap doesn't remove dirt, just sort of locks it on. Which does not matter as much on skin but is disastrous for hair. I have known a couple people who had to cut their hair off after trying to wash it with soap.
If I was to the point where I could only afford one, I'd get shampoo. If that was impossible for awhile, water only is the way I would go. It works ok, most stuff rinses off.