Your not wrong Noseall, a lot of companies get the fitters to build the base or pay builders very poor money and dont give a fig what they do to make a living, I know cos I've worked for some! The last lot I spoke to, Britelite in Strood Kent, offered me £150 a day for me and a lad, use my own van, on a time limit of say 3 days for an average size conny so we have to bust our balls to make a living.. I declined.
The material cost is nigh on double for a conny over an extension, especially a glass roof, mine was 4.1 x 2.1 and cost me £1,100.00 add on top the greedy company salesman on 10-15% the company its self making maybe 20% and its not hard to see where the money goes..
I do actually build my bases as 'like' an extension, both skins built off the foundations that are at min, 600mm x 450mm, all correct damps, 150mm over site with insulated walls and slab, S.S. ties etc