Being a boring sod, i had a wander over to my local BCA auction last week, my old company car was going to through there, and i was intrigued to see how much it would fetch at auction, lex wanted 10.4k for me to buy it direct from them, at auction it went for 9.1k. I would expect the car to be sold for around 13k (91k miles Avant TDI with twinkly lights and few olther options).
I felt most cars were quite pricey considering its a trade auction, so the mark up cant be all that much, i guess its all about the volume. Merc A180 CDI and BMW 118D were being snapped up like there was no tomorrow, same for any SUV type motor.