Unfortunately there is no safe, humane and legal way to kill your dog at home. The solution the vets use is very carefully mixed and controlled to ensure as best as possible a quiet rapid passing.
In some areas it is legal to shoot your dog which can be humane if done properly.
Many of the meds you might think of to use for an overdose will cause the dog a great deal of distress.
You can check with the local humane society to see if they offer this service.
You can call local vets to see what they charge for euthanasia and your choice of how to handle the body.
If she does pass at home you will still need to decide how to handle her body as depending on where you live it may or may not be legal to bury her.
As cash flow is an issue you can also check to see if any of these groups will help
here are some groups that might help you afford the vet bills:
American Animal Hospital Association
" 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
"Our services range from financial aid to complete treatment
to those pets and pet owners in need."
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
God's Creatures Ministry
"This fund helps pay for veterinarian bills for those who need help."
"Our efforts focus on serving the elderly, the disabled, and the
"We are dedicated to insure that no
companion animal has to be euthanized simply because their caretaker
is financially challenged."
The Pet Fund
"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
"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
Hope this helps you!