Interesting thread. I’ve measured soap/cream per shave quite a few times and my experience is very similar to
@Matsilainen I do a 2-pass shave.
I’m averaging 0.5g to about 1.5g per shave. Here are some examples.
MdC - tub lasted 9 months shaving 6 times/week, lathering with a Chubby 1 Super, averaged about 1g/shave. With a more modest brush, I can get by with as little as 0.5g MdC. It’s a soft, but very concentrated soap.
Castle Forbes - I measured it out, 0.5g/shave, consistently. That means that there is about 1 year’s worth of shaves in a tub. A gentleman on another forum calculated that CF cost him about 0.5 cents/shave over Proraso.
Other concentrated (~0.5g/shave) soaps/creams are Acqua di Parma (current yellow tub/tube version), Santa Maria Novella, ABC hard soap (discontinued), I Coloniali Mango Oil soap (discontinued), Pre de Provence. Most good quality hard soaps and a few concentrated creams can be close to 0.5g/shave.
i’ve also learned some things. If the soap doesn’t lather well or takes excessive loading, try a different brush. A lot of (especially) North American artisan soaps are not cured very well and contain a lot of water, one of my peeves with them. If you’re using >1.5g per shave, you’re probably buying water at soap prices. They can be VERY expensive to use because getting 0.5g of soap required 1.5g (or more) of product. I prefer concentrated soaps and creams for the economy of use, plus they tend to be higher quality products.
Hope this helps!