Good chance that flexible PCBs are so little in demand that it requires special setup to make your prototype. I've seen them in quite cheap products so mass production (with that setup cost spread over thousands of units) is much cheaper. They're indeed VERY expensive in small volume.
Part of the reason they can make the regular <100x100mm PCB so cheap is because those are panellised into much bigger boards together with orders from other people and then mass produced, no special machine setup to be done.