The cost varies to do this. If she has to be put down tonight to spare her pain you can check here for an emergency vet
http://www.vetlocator.com/hotline.php
I saw three listed within 25 miles of you and this one was the closest seeming
ANIMAL HOSPITAL OF NASHUA 168 Main Dunstable RdNashua, NH 03060XXX-XXX-XXXX you can call and ask them for costs of doing this. Emergency services are much costlier usually than your regular vet is but they may make an exception for euthanasias.
If not then this section of the above site can help you find a local vet and you can check their hours by calling them and seeing if hours are on their answering services
http://www.vetlocator.com/search_results.php
in your immediate area there is even a mobile vet.
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics