What do you class as ridiculous prices and what did they quote you for? You need to ask what size uprights will they be using, what size beams for frame and roof, what decking (different thicknesses, widths and pattern). When I built mine I used railway sleepers for the uprights, so they probably cost 2*-3* as much as 4" posts and I notched them all to slot in together. I used 6"*2" for all the frame. I know people that have had roughly the same size as yours built for about £1000, but it was built out of 4" uprights, 4"*2" for the frame and thinner decking.
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