Call-out costs vary enormously. If you regularly take your car to the same little independent workshop just around the corner from you, and you ask the owner to come and jump-start your car, he might say "Just call it a fiver". (You'd be fairly lucky but it does happen! I've got a local garage that probably wouldn't want much more than a tenner).
More often, though, you'll find that there's a minimum call-out charge (which, typically, includes the first half hour's labour). I guy I knew, who used to manage a car workshop in Felixstowe, had a vehicle break down directly acoss the road from his garage. The driver phoned his breakdown service and they phoned the garage. The mechanic strolled over the road to take a look. 5 minutes later he was back in his office having sorted out the problem in an instant. He was also very happy because he could charge the breakdown service his minimum �50 call-out fee! This was over 10 years ago. That fee would now be nearer �100!
So it's worth forking out the annual premium for breakdown cover but there's a much better option than Green Flag: I've always used Autonational Rescue. (Cheap rates and excellent service). Their complete package (roadside assistance, recovery to home address & home start service) is �84 but you get 25% off if you haven't claimed in the previous year. You'll get that discount when you first join - even if you've called out Green Flag, or someone else, a dozen times in the previous year - so you'll only pay �63 for the first year and you'll keep getting the discounted rate if you don't call them out.
You can find Autonational Rescue here:
http://www.autonational.co.uk/index.htm
Hoping this helps,
Chris