I think it is about right, because my friend who has paid £3k for hers. We josh each other as we both have to pay decent labour costs for things that our not in our skill-set!
I don't know how wide your patio windows are - but at least 2m length. Adding together sun proof lining and bump for insulation, you are looking at about £30 a metre for decent fabric, and more for posh.
I am about to make curtains for one of my children's new homes. I don't know long it will take - my arthritis means I work in fits and starts, but proper measuring & cutting takes a lot of time, before you start on sewing.
But also never doubt the investment. I have finally given away 45 year old curtains that were in our sitting room for 35 years, and have done the round of relatives ever since. The fabric has not faded, they still offer good insulation, and look great.
I type this in front of curtains that are 20 years old and that I remade 10 years ago to fit this window.