Id like to point out, that if they are making a decent wage it's almost certainly illegally in context to Cuba's laws. (My wife is from Cuba, she has many of her family still living there)
I know very little about Cuba, but given it's been cut off economically from most of the world, it's no surprise that currencies would be different down there.
According to Numbeo.com [1] a cost of basic utilities (Electricity, Heating, Water, Garbage) for 85m2 Apartment is ~$6.09/month. A one-way ticket on local transport is $0.04. I've picked two that jumped out at me as crazily cheap compared to places like the US/UK, but looking down that list I don't know if living on $17/month is doable. But without fully understanding the environment/economy... it's a lot more complicated than just thinking that $17 could never be enough to live on.
Generally It's not enough to live on. However there is a thriving black market, prostitution or just "hooking up" with wealthy foreigners is common. For example a bartender at a resort will likely have his/her own bottle of alcohol behind the counter and sell that while pocketing the cash, later when they check the alcohol everything lines up for drinks sold vs revenue but the bartender skimmed $x in revenue off the top. Underground capitalism/entrepreneurship is how people get by (risking serious jail time daily).
But there is a subset of the population earning good tips working in the tourist industry and receiving funds from family living outside of Cuba