S&L
I don't think its a question that can be answered very quickly! Look at section H of Lasors for all the requirements to be a flight instructor.
To be a paid FI you need a CPL, 200 hours total time and the CPL knowledge exams passed to start the FI course, and the FI course itself. Depending on where you do the training, you could be looking at £4-6k for the CPL, the same for the hour building and the same again for the FI course, costing roughly £15k in total, not including living expenses. On top of that you'll have the CPL exams to do, which will cost in the region of £2k by distance learning.
People always say to add on a percentage for over runs, exam fees etc, so say about £20-22k total. That's just a ball park figure - but you did say it was a quick question, so here's a quick answer!
Hope it helps
PW