On Cb, a lot of people actually also love the fact they can make themselves stand out using different coloured fonts, or font styles with the supporter package, but I think the main thing is for the PMing.
You can tip as "anonymous" by being a supporter, too but honestly, I don't really see many send tips as anonymous on CB.
Even with your maths on saving/spending, I think the $21 spent on the supporter status is "invaluable" because they can mostly get around the whole "PM costs XX" and while a lot of models will tell people to tip for the PM / it's on the menu, a lot will still reply to the supporters who first PM them. So they get unlimited PMs basically, so it's invaluable