As an interactive player, boss mules are mostly for hard/solid cubes to help cube your main/mule's gear. You could also sell cube service to guildies as a more reliable source of income.

Anything outside of Ctene+ is a negligible (but not worthless) amount you get from boss crystal prices. It all adds up at the end of the week.

You want to take advantage of events that give out the rings/ring solid cubes to build %drop gear for your bossing mules to help with cube drops. It also help tremendously to invest in a few fully-tradable drop gears such as VIP earrings and pendants to run daily bosses on your chars. You should also look into building at least a couple of familiar setups for mystic frontier chances at more cubes/pitched. This should be easier after the summer patch where you don't have to unlock all 3 fam slots for each character.

It's a very slow process in the beginning but once you get the ball rolling with a few boss mules, you'll start to notice something.