Same cost as cars and houses. Bigger and better they are the price goes up. On topic, my Penistone Reds one made about ten years ago from Hampshire Flags was top quality material, eyelets, and lasered on text and images. Got cheap as knew someone there but you’re looking at three figures for one not to be laughed at.
This is where we got the Patrick Cryne flag from... https://www.footballflags.co.uk/products/design-online.html