My sympathies on your need to make this decision.
Prices vary vet to vet but you can call and ask what your vet will charge.
Some costs for handling the body can vary too depending on if you want to bring the cat home for burial, have the cat cremated, or have the vet handle burial etc.
My vet charges $50 to give the euthanasia solution but again that cost varies area to area.
You can also check with your local animal shelter as some provide this at a reduced cost.
Hope this helps you!