I think the cost depends on where the OP lives in the world.
There'a alot fo people who say "oh, that's excessive ... etc" and then say what their cat neuter cost.
Down south surgery rents/wages/blah/blah cost alot more- so if OP is in Central london then maybe that's not so unusual....
and i would say that letting an un- neutered cat roam free is not a good idea. The cat is more likely to get involved in fights, damage other catss and get damaged himslef.
he is more likely to catch FIV (simailr to HIV) and to get other cats pregnant.
Unneutered cats are more likely to get involed in road accidents in which they get hurt or people get injured whilst trying to avoid them- they are just more likely to roam and get involved in more sscrapes etc.
Retained testicles are likey to become cancerous and twist round and cause BIG problems later in life.
and not to have any ID isn't the best idea.
But- the man should havhe asked around and found out if anyone owned the cat before taking him to be neutered.
I still can't work out whether or not he has asked you to pay- maybe you can get a breakdown of the bill- ie blood tests pre op and decide what you would have paid had you been deciding on the surgery...
I'm not sure the best way forward with this- but he has done your cat a favour by sorting him out...
and to the poster who says tht the cat doesn't need more than a long acting injection- you don't actually know what happened duriing the surgery so you have no idea why the cat is on abs- so please don't decide over the internet whther or not somebbody is treating the cat correctly or not