I realise this is the cheap skate option.Amazing render an a great use of space, but as someone who just had to fork out getting curtains for two 3.6m wide windows i'm wincing at the potential cost of those curtains, especially if you're going for a x3 gather as what looks like in the renders. Worth keeping that in mind in the budget.
like you I was horrified at the cost of near ceiling to floor curtains to cover a set of 4m bi-fold doors
in the end we bought 4 pairs of curtains similar size to these
Buy Teal Velour Eyelet Curtain Pair 229x229cm at TK Maxx
like I say, cheap skate option but, very nice quality curtains at about 70% cheaper than the quick online quotes I was getting.
it does help that we bought very dark grey and the gather is about x3 so the join really is not noticeable. I would imagine with light curtains or low gather this would not work