When one of mine was PTS at the vet a few weeks ago, he charged £17. This was for the consultation, injection,and disposal. I could have taken her body home with me but that wouldn't have made a difference to the price. This seemed very reasonable to me, as I would have hated to have had the experience of killing her myself as my last memory after 5 happy years with her. It would be good if you could find out how to cull a hen yourself in an emergency, though, if a fox attacks for instance and a bird is in severe pain and shock, unlikely to recover, and it would take too long to get her to the vet. Or if you find a dying pheasant that's been shot, if you live in the country.
If you haven't already done so, you might like to enquire around local vets to find one that is fairly experienced with chickens, in case you need one in a hurry. I'm my experience, however, so long as chickens are kept safe from fox or dog attack, they either tend to die suddenly from unforeseeable heart attacks etc, or have more long-drawn-out illnesses which give you time to think what to do, ask the knowledgeable people on here, and try to avoid the vet as much as you can as their fees for actual treatment can be very high and some common problems do respond well to home remedies. An experienced chicken vet can be hard to find, and most vets will just tend to prescribe broad-spectrum antibiotics, but you need to think carefully about going down this route, even with a much- loved pet. See thread in Chickens Health 'Is your antibiotic really necessary?'
http://poultrykeeperforum.com/viewtopic.php?f=7&t=8760
It's just my personal opinion, and I'm sure many people will shoot me down in flames if I say that hens are actually quite short-lived pets, and there's no point in trying too hard to keep them going when they reach the stage of succumbing to common age-related illnesses such as peritonitis or prolapse. The difficult bit comes in recognising when to let go, and what is the best way for you to end their life, and then your very sensible and responsible enquiry about disposal of bodies is part of your decision.