Never ever give your dog human medication unless it is prescribed by a vet, definitely not co codamol. £200 to pts a stray cat is extortionate and the alternative to put him back on the streets is ludicrous. My vet takes in and treats strays at no charge, and I have also taken him other wild animals too. Time for you to find a new vet I think.
You could try a herbal alternative to a prescription medication, there are numerous products available ie: Yumove.
http://www.lintbells.com/products/yumove