On extinguishers if you are doing well and at a reasonable company the most you will earn will be about £25-29k.
Quite a few companies I know pay around £12500 plus commission of around 8% to 15%.
I agree with the others here, I would love to be able to charge our customers a lot more so I could pay our engineers more, but with more and more Ex Firemen etc setting up and charging as little as £24.50 for a new extinguisher (this is happenng in my area, before anyone starts moaning at me) the arse is falling out of fire extinguishers.
I have recently seen a leaflet from a company called Lightning Fire from Doncaster offering to service upto 4 extinguishers including all spares and refills for £17. Charging that sort of money I think £50k is not going to happen