I've done some consultancy work for another aspect of 'clinics' in the past and it is true there were a lot of nurses involved. Chatting to one nurse, she said she supplemented her cosmetic work with occasional bank shifts, so that could be an option, this could well have been as a way of maintaining her registration, as well as to earn money.
I have no idea who makes what money and I'm sure some don't make very much, others make a good living or indeed a fortune and it will depend on how good a business person you are and whether you have a good source of clients locally, plus what the competition is.
You should be aware that it appears that regulation in this area is being strengthened so something to investigate further before taking the plunge. This is the area I'm more interested in and in my limited experience not all operators offer a safe good quality service.
But then if that's your aim, increased regulation could well be a good thing, as it might force out some of the less reputable providers, who likely undercut those who aim to follow best practice.
www.gov.uk/government/news/government-to-crack-down-on-unregulated-cosmetic-procedures
It was also well known that some of the apparently successful clinics were actually fronts for money laundering and organised crime etc, so don't take everything at face value. I'm not saying this is the case for your 'botox lady' but it is for some of them.