Hi Cheryl...I'm so sorry that you are having this experience with such a great breed of dog.
I have an Amstaff and a Pitbull that have been treated for Lymphosarcoma, but have recently had a Pit mix pass away suddenly at age 4 years from Lymphoid cancer in his intestines...he gave no signs until the cancer had invaded the entire digestive tract. I do know what you are feeling today.
Since the acute form of these cancers are so aggressive, and since the dogs most often afflicted are not complainers, we get ambushed with difficult decisions and feelings.
You cannot make a "wrong" decision at this point...if she can hold on until you can get an emergency appointment with a Veterinary Oncologist, then you can get some more information about treatment, etc...do attempt to locate this kind of help ASAP.
If she continues to go downhill this weekend, you would not be unkind or unrealistic to let her go.
This link gives the prognosis for acute (what she seems to have) and chronic leukemia (both of which are poor):
http://www.gopetsamerica.com/dog-health/canine-leukemia.aspx
Here is another link to a site maintained by another owner...it gives a link to an online support group:
http://www.haileybell.homestead.com/
Costs for treatment is not usually outrageous...the medications used are pretty easily obtained, but the administration must be done carefully as these are toxic chemicals. Specialty groups do include these protective measures in their charges, and costs do vary depending on the cost of living in your part of the country.
If there is any other support that I can give you at this time, please click "Reply".
You are both in my thoughts.
Your concern has touched me--if you have follow-up questions, or need to clarify your case, please let me know right away! Your "Accept" helps me reach out to others!