Please note that the FP3 has never cost 500£, not even when it premiered four years ago (it started at 450 Euros, I believe). I think with a little attention and patience you might find a used one now between 100 and 200 Euros.
Fairphone has pledged to continue software support for the FP3 until August 2024 and is aiming to add two more years on top of that. Custom ROMs might support the FP3 even longer.