If your house has always had upvc fascia boards from new the chances are the wood underneath is in good nick and you can go over existing with 9mm capping board as most do anyway.
If your original fascia boards were painted and you think they are suspect do as joss has done.
Are you going to replace the sofit and barge boards at the same time!
You may find tis useful as a guide:
https://www.checkatrade.com/blog/cost-guides/fascia-soffit-guttering-replacement-cost/
There are also specialist companies who clean fascia’s etc. Providing they haven’t yellowed (if white), which I very much doubt, they will come up like new at fraction of the price.