One of the good things about wanting to become a driving instructor, is the fact that once you have qualified, it means that you will have a government recognised qualification. This basically means that it doesn't matter if you move from Cornwall to Scotland, you would be guaranteed a job, either by putting an advert in the local paper or taking on a franchise from a driving school.
The cost of driving school franchises varies from place to place. Some of the big national driving schools can charge up to 360 per week. However you will find that most charge between 30 - 100 per week. There are generally 2 types of driving school franchise, Own Car and Full franchise.
Own car. This is when you have dual controls fitted to your own car and the driving school provides the graphics for your car or magnetic signs and the top box, as well as your pupil leads.
Full Franchise. The driving school provides everything including the car, the insurance and all your pupils. This franchise is between 180 and 360 per week, but is ideal if you are just training to become a driving instructor and can't really afford to set things up on your own.
However a word of caution for those taking driving instructor training, beware of franchises that say you can leave the firm in as little as 3 weeks if things don't work out. Why? Well would you go to the trouble and expense of advertising for pupils, providing graphics and cars for a driving instructor and then just let them walk away? It wouldn't make financial sense! So chances are all they are going to do is set you up on a franchise and your franchise money will pay the advertising, so they in effect lose nothing.
In which case you may as well just done the advertising yourself. If a company wishes to tie you in for say 6 or 12 months, as long as you have a clause saying if work is not provided to a minimum of hours, then no franchise is due, you can be fairly certain they are going to invest time and money in you and they want to make sure you don't just reap the benefits and walk away. Which if it was you running the company would seem fairly simple business sense!