I know as a CB Subs tester that, in the beginning, before I dove in myself, I had absolutely no clue as to the abilities and features this gem provides. Since then, several plugins have been added to extend usage even more.
I've been too busy testing to get my business off the ground, but I consider it such a time-saver (and face it peeps... nowadays, time is money and a very precious commodity) that I plan to implement it on at least two other domains, even if it does take some saving. It's a one-time payment for the license, and maintenace is cheap.
I've looked at and tried other "shopping carts" and ACL extensions, some of which cost much more when I included the cost of the addons I wanted. Sobi2 is free, but after adding up the paid addons, I decided it wasn't for me.
Other components simply don't come close to the control CB Subs allows. Integration is built in, so you don't need bridges or hacks! The ACL control doesn't hack Joomla's core as most, if not all other User Control extensions do.
Frankly, I was expecting CB Subs to be at least double the cost, with yearly fees (and I don't mean just a $20 maintenace fee either!). If you compare prices and features of other extensions, I highly doubt you could find anything that even remotely compares.
Think about it this way...
If you use The Starter Plan for only one year and find yourself too busy to keep it up, you've lost the cost of a couple foo-foo coffees or two fast-food lunches per month. Not only that, if you continue to use it, upgrades are only $20 a year, and one customer (or one family visit to McDonalds) can suck that up.
Go ahead, read the features, compare and make up your own mind...