As microchipping has to be done by your vet, it will cost you the vets call out fee plus a charge for the microchipping. When the passport scheme was introduced, our vets offered a special deal on microchipping & passport - I think it was aroung £20.00. Otherwise you can get a form from your passport issuing office and the vet will complete it at the same time as doing the microchip. It will probably cost you around £25.00 plus the call out fee.
If you can hang on can you not combine it with the booster? We are lucky as the vets are about 5miles away & it is cheaper to take the horses to them for things like this. £25 is about right,