I believe they were yellow S76s. They stopped flying when the M25 opened all the way round from LHR to the M23 junction, thus eroding their time advantage. I have no idea how much they cost but I recall that it was seen as expensive. The locals didn't like the noise either, and I recall that the service was seen as a stop gap.
When I were a lad there was a service from LAP to Battersea by S55, but it was little more than a token, and faded out.