If you are able to do such relatively straightforward work yourself, fitting new discs and pads is a fairly cheap maintenance job on smaller cars. It's only when you get to larger and heavier cars that the prices tend to get a bit juicy. I recently helped a friend fit new pads all round on a Merc C class estate, the pads and wear sensors alone were £100, discs were still comfortably in spec but would have added a further £200 and he doesn't intend to keep the car much longer so just did the pads.
A set of front discs and pads for cars like the Jazz, Yaris, Fiesta etc is usually comfortably under £100 from motor factors. I like to use Brembo where available, but I'm sure other brands available from reputable UK motor factors are perfectly OK. At this price point I don't usually mess about with old discs, just replace the set.
As with tyres, I tend to replace brakes earlier rather than later. £100 for brakes is trivial compared to usual fuel/insurance/tax costs over the same distance.
