I've been lurking the threads around here and there's something that confuses me. Is there any point to using other oils, say almond oil, grapeseed oil, avocado oil and the like, if soap only cleanses? Same for using milks and such in place of distilled water? These kinds of ingredients tend to be more expensive. If they don't do anything for the skin, why make the investment? Or is my understanding too simplistic?
Not simplistic...it takes time, and there is always something new to learn.
Yes and no. Using expensive oils/butters is a waste of money unless you are going for 'label appeal' because the majority of 'benefits' are destroyed by the caustic nature of Sodium Hydroxide/Potassium and the saponification process which breaks down those oils/butters into fatty acids and glycerin. Additionally, soap is a wash on/rinse off product that is only on the skin for a very short time...five, maybe ten minutes. It would be like putting on hand lotion and then washing it off. However, it's those very fatty acids that contribute to the 'soap qualities' that we strive for. It should also be noted, that with some exceptions...like Castor Oil...using an oil or butter at less than 10% isn't always worth it.
Sometimes the water replacement is for label appeal (IMHO)...like when using a vegetable puree. It's pretty cool to make a Pumpkin Pie Soap with pumpkin puree, but it doesn't 'add' anything to the soap. Using coffee/tea or fruit juices or beer or bourbon also doesn't add anything and in fact, can be a bit problematic given the sugar content of juices and alcohol. Milks (goat milk, buttermilk, coconut milk/cream, heavy cream, yoghurt, etc) are different because of the sugar and fat content and there is a huge following for Goat Milk Soap (it's the reason why I got into soap making). There is a gal I know that she pretty much only uses goat milk or aloe vera juice. I've never used the latter so I don't know, but I know that there are folks who love it too.
As to cost...it depends. If you have goats and cows, then it's fairly low cost. You can also purchase powdered milks and you can also make your own 'milks' like almond, coconut and oat. I was getting my goat milk from a local source for around $5.00 a gallon...not as cheap as distilled water but folks don't mind paying a little more. I haven't used coffee or tea, but neither is very expensive. Alcohol can add to the cost but 1) you generally don't do a full water replacement and 2) you're not going to waste expensive alcohol unless you are going for label appeal and you can pass it on.
I make my Regular Soap with Olive, Palm, Coconut and Castor Oils, and Cocoa and Shea Butters. Now I can make a nice bar of soap with just the oils and it would certainly be cheaper, but I really like what Cocoa and Shea Butter brings to the table and so I use them. It isn't always about cost because if it were, commercial soap is much, much cheaper.