to replace the oil seal will take around about 4 hrs I reckon. And you may as well change the cambelt as well as all of this has to come off, mind you yours is probably oil soaked anyway.
the seals themselves and the o'ring are quite cheap, the most expnsive one is the one that goes in the timing cover I think at around £30 depending upon year!
for a garage to do I reckon your looking at acouple of hundred quid at least.
My camshaft oil deal went after dunkin my motor in some very muddy water without the wading plug in - I ALWAYS use mine now!
until its done , dont leave the wading plug in