Luna the Sick Budgie

This April, my sister and I adopted a budgie from someone on Facebook, on behalf of his friend who couldn't keep it any more, the owner said that she had two budgies and one of them died, which forced her to find another home for her other one. We were so excited to give this little bird a new, loving home, and brought her in straight away. At first the budgie, which we named Luna, seemed stressed, as one would expect, having just lost her partner and now moved to a strange place. 
A few days after rescuing her, we noticed her tail bobbing up and down, which is often a sign of respiratory infection, and took her to the vet to find out she might not survive the next few days. She had been sick for a long time (her sleeping situation equivalent to us sleeping on a pile of clothes on the floor, for two years) and we were told to put her on antibiotics for 10 days. 
Giving her medicine made her upset, but we moved her to a bigger cage with better branches and she pulled through that first week. 
About a month after her first vet visit, we found blood in her droppings and we don't have enough money to pay more vet bills (we're both full-time students). We've had to borrow money to pay already but we have to repay that as soon as we can. 
Some people have asked why we took in a new budgie when we can't afford the vet treatments, but when we adopted her we were under the impression that she was healthy and would not need to have multiple emergency vet visits. We just wanted to help another person and an animal in need. 
The night after finding the blood we had no choice but to take her back to the vet, and he said she might have dangerous mega bacteria in her stomach, or metal toxicity from zinc or lead). By the amount of blood, he was surprised she is even alive. He kept her at the clinic in order to give her special treatment for the next two nights but having her stay at the vet cost us $70 per night, on top of consultation and medication fees. The whole visit ended up costing $300, with the previous visit costing $150, and having to rent an incubator to keep her healthy for $400. It's all adding up so fast, her treatment isn't even over, as the vet hasn't come to a conclusion as to what it was (tests for mega bacteria and metal toxicity came back negative) and it's money that we honestly just don't have. We want to do all in our power to ensure that Luna has a long and happy life, but our current savings just don't stretch that far.
It's been so hard rescuing a bird assuming it's healthy, then to find out that it mightn't survive a few days. To spend more money on different cages and treatments and other things that might help her, only to wake up one morning to find that she is no better than she was when she first came here. 
Some people we've spoken to said they would be happy to chip in, and we wouldn't ask if we were any less than desperate. We know that this might be close to the heart for some of you, so if you have a dollar or two to spare, the three of us would be so incredibly grateful.