My wife made ours, but I would say spent ages on them as they are pleated and took an absolute age to make and stich them correctly, she had the same issue the cost was high, I would gather because of the time more than anything else...
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Up north where we are we got some run up at 'In Shops' for the price of the material and a few quid.
Simply gave the old curtains to the nice lady and she rang us back a few days later to say they were done!
From memory about 30 quids worth of material and that stuff that holds them on the caravan curtain runners and about 60 quid in labour so a whole set for a 4 berth van for just on a 100 pounds.
My wife has recently made a set for ours and i reckon she's easily spent 16 hours in doing so. Add to that cost of materials and its sounding like realistic quote, cheap if anything.
------------- I went to the top of the mountain...and looked over the other side!
For our campervan I bought some ready made ordinary household as near as possible to the window sizes ones and cut them down to size. Easier than making from scratch if you aren't a confident sewer.