Depends.....

None of my pcps (touch wood) have needed anything other than an occasional pull through or external clean, some are many years old.
If you are confident/competent you could maybe work on them yourself, (eg: see AGF forum, google and youtube vids for guidance).
If not so confident, there are many on here and elsewhere who can work on them, incl many RFDs.
Obviously every gun is different, some may go years and years without the need for any work, and others have issues straight from the shop/supplier, just like every other device/appliance, car, toaster, TV, etc.
Cost would depend on the gun, its condition and whats wrong with it, could be £50 or it could be £250?