Seized brake caliper

you can buy service kits for a few quid as long as the mechanism for the handbrake still springs back nicely.

lazy owners tear out the dust seal's to unseized them "once".

That price sounds in the right ball park. Most callipers are "remanufactured" and require you to exchange the original. The company on the attached link is popular - selling Budweg remanufactured callipers.

A seized calliper is simply where the seal has twisted & jammed either because the side of the brake piston is rusty and/or the seal has hardened, aged of become damaged. Refurbishment kits are cheaper and involve new seals and dust covers and if necessary new pistons - assembly can be a bit tricky if you haven't done it before but there are several "how to" You Tube videos. A straight calliper switch is much easier and quicker and is normally what garages prefer.

http://brakeparts.co.uk/#!/

Additionally make sure you get a tool to twist out and in the pistons - but the be fair if it is seized it will be a struggle to twist it out and if you use the handbrake mechanism and it is seized really badly you will damage the mechanism as it is mechanical and moves it a mm or so on each action. I serviced both of my rear calipers and that was the first time I had ever done it - the kit was a tenner. but one side should have been replaced as the handbrake arm did not return with much gusto and that is a non serviceable part.