Currently in order to earn money from flight instruction you need a CPL. This looks to be changing with EASA, where you could earn money from a PPL/FI, but you would still need the CPL groundschool.
If you only have a PPL you need 200 hours to do the FI course, so the saving you make by not doing the CPL is lost in building an extra 50 hours.
The FI course is currently about £7k, but it usually seems pretty difficult to get that first job. I was in full time work with regular students perhaps 6 months after qualifying, but that was 6 months of scratching around between 4 local schools doing trial lessons and trying to get noticed.
You need to be supervised for at least the first 100 hours, some schools are more open to this than others, and usually pay restricted instructors less. During these 100 hours you also need to send some people solo, and note down 25 examples for the CAA.
Once unrestricted you can send people on their first solo, as well as first solo nav, and hopefully a nice pay rise (mine was in the order of 25%).
If you're looking to change to a career in instructing, be aware that the take home pay will be in the order of £12-15k, so not a lot for your investment. If you go on to be a career instructor with plenty of experience I'm sure you could negotiate something better, but for basic PPL that's about it.