The medical takes about an hour and is fairly similar (from the patient's point of view) to an insurance medical or BUPA health check. Basic checks on breathing, lung capacity (blowing into one of those tubes), heart function (with EEGs depending on age and any abnormalities found in the heartbeat), hearing, eyesight, reflexes etc. I think there was a blood test but have forgotten. You are asked for a urine sample (so, if you want to acquire the pilot's prerogative of shooting lines it's best not to, er, shoot any lines, if you catch my drift).
Anyway, nothing particularly complicated. Costs apparently vary rather a lot depending on where you go, but seem to be in the range £100 to £250 (I think I paid about £150 for mine: pleasant doctor with lapsed PPL doing aviation medicals for KLM and other airlines as well as PPL studes, based in Harley Street).