need medical advice EXPERINECED OWNERS ONLY, PLEASE!

I took cloud to the vet the other day. The diagnosis isn't good.

Cloud's white blood cell count is way above what it should be where as his red blood cell count is very low. he pees and dirnks way too much for normal, so the doctor is suspecting that it is kidney disease. Cloud also has very bad rotted teeth with very little gums. The main reason i took him in was because of the large glof ball sized lump on his right hind hip. It is getting worse and has now extended down to his leg and foot. The doctor suspects it is either a very very bad infection or cancer. He has given me strong anitbiotics and pain meds to administer to cloud for the next two weeks. thet vet is wanting to do a biopsy.

Now here is my problem. I am at my financial limit. In the last 6 months i have spend over $1100 on this little guy with this last vet visit costing me $635. I now have no rent for next months, not that i'm complaining, just that i am at my wits end. I cannot afford to the biospy and i just feel like a horrible person. i strongly believe that you should not buy a pet if you cannot take it to the vet. i feel like i have failed Cloud because it is not his fault, what i need to know is whether or not to put him down now before it gets too bad or keep him on the meds until i feel it is right for him to go. there is no other option.
i realize i'm going to get crisized for my decsion but realize that whatever you choose to say to me i've already said to myself in a much meaner way.

thanks for the advice.