For general use (and especially as a drying aid after washing the car) I use the Meguiars Last Touch detailer, which I buy in bulk (1 US gallon or 3.78 ltr) containers.
I don't think its quite as good as the Gold Class version mentioned by the OP, but following a tip off from another forum I bought some today from Euro Car Parts - its normally £28.90 delivered for 3.78 litres but if you use code bigdeal31 you get 31% off, so £19.94 inc delivery.
http://www.eurocarparts.com/ecp/p/car-accessories/car-cleaning-accessories/exterior-cleaning/car-detailer/?549775901&0&cc5_761
Bear in mind you dilute it down 1:1 to use it, so it will give you 7,650 ml - or in other words 16 * the normal 473 ml size bottles of detailing spray which works out at less that £1.25 each!!
I've no idea how long the code will work for but good luck if you fancy some.