First of all, someone’s main motivation for teaching yoga should never be financial 
Sorry, but based on my experience as a teacher who has qualified in last few years (& still with full time job), it’s not a money maker for most. Especially community classes.
However, it IS an amazing, fulfilling role and pursuit to undertake in order to share Yoga and see others benefit.
I know a good 20-30 teachers who have trained in the last few years. Only a handful teach yoga full time. Most have adopted it alongside their job and look to develop this over a long term period eg shifting to part time work to teach more yoga, or teaching more when they retire.
One of my classes is community type, in a community centre. So, it’s priced affordably at £5 per person. I had 8 students last class, take away £15 for Room hire, then think about tax, Travel etc. I also do some work for studios, tend to be bigger classes and you can get paid approx in the region of £25-40 per class. Again, factor in taxes, Travel, time to plan classes.
It can depend where you are regionally, how saturated the yoga market is and how good you could be at marketing yourself and your classes.
There’s a Facebook group called Yoga Teachers Uk, if you join and read posts you’ll get a nice overview of teaching as a job. Talk to lots of newly qualified teachers if you can. Also a relatively new book which is useful: www.amazon.co.uk/Yoga-Teaching-Handbook-Sian-ONeill/dp/1848193556?tag=mumsnetforum-21
I’ve spent in the region of £9000 undertaking various courses and training over the last 6/7 years. To me, that’s been an amazing investment in my own self development and Health. I hope that one day I will make more than a bit of pocket money from it, but if you could be content to invest the years training, and see it as a real long term part of your life, then absolutely, go for it!
Namaste