Answer

If this is an emergency vet then call your regular vet and ask about the costs of amputation surgery.

I can tell you last year I paid $2500 at an emergency vet for an amputation on a cat sized dog that my regular vet (who was unfortunately out of town) would have done for $800 or so. That was a front leg and rear legs can be trickier depending on where the amputation will be done.
Three legged cats can do fine.

You can check out Care Credit for financing surgery. If you qualify this may work well for you.
http://www.carecredit.com/
A credit card company for health care, including veterinary care.
"With a comprehensive range of plan options, for
treatment or procedure fees from $1 to over $25,000, we offer a plan
and a low monthly payment to fit comfortably into almost every
budget." They have a zero interest program if you pay within a certain time frame and they list the vets who work with their product.

Charitable giving is down but you can consider trying to find financial aid.

cash flow is an issue, check with your local humane society to see if there is local low cost vet care, ask your vet if you can do time payments or here are some groups that might help you afford the vet bills:

American Animal Hospital Association
http://www.aahahelpingpets.org/
" Through the AAHA Helping Pets Fund, veterinary care is possible for sick or injured pets even if they have been abandoned or if their owner is experiencing financial hardship."

Angels 4 Animals
http://www.angels4animals.org/
"Our services range from financial aid to complete treatment
to those pets and pet owners in need."

God's Creatures Ministry
http://www.all-creatures.org/gcm/help-cf.html
"This fund helps pay for veterinarian bills for those who need help."

Help-A-Pet
http://www.help-a-pet.org/home.html http://www.help-a-pet.org/
"Our efforts focus on serving the elderly, the disabled, and the
working poor."

IMOM
http://www.imom.org/
"We are dedicated to insure that no
companion animal has to be euthanized simply because their caretaker
is financially challenged."

The Pet Fund
http://thepetfund.com/
"The Pet Fund is a registered 501(c)3 nonprofit association that
provides financial assistance to owners of domestic animals who need
urgent veterinary care."

United Animal Nations
http://www.uan.org/lifeline/index.html
"The m ission of LifeLine is to help homeless or recently rescued
animals suffering from life-threatening conditions that require
specific and immediate emergency veterinary care. We strive to serve
Good Samaritans and rescue groups who take in sick or injured
animals. In certain cases, LifeLine can also assist senior citizens
and low-income families pay for immediate emergency veterinary care." They also keep a list of local and national help resources here
http://www.uan.org/index.cfm?navid=163

 

More groups here http://www.speakingforspot.com/helppayingforveterinarycare.html

 

Feline Veterinary Emergency Assistance (FVEAP)
http://www.fveap.org/sys-tmpl/door/
"Seniors, People with disabilities, People who
have lost their job, Good Samaritans who rescue a cat or kitten - any of these folks may need financial assistance to save a beloved companion."