My best friend needs help

Hello everyone. This is my cat, Katie, who I've had since I was 6 years old.
A couple weeks ago, I noticed that a hard lump formed on her back. I wasn't sure if I hadn't noticed before, but it was roughly the size of an almond. I of course, started to worry, because she didn't have anything like that before.
Katie has never been desperately ill before, but after that lump appeared she's been more lethargic, and has constantly had issues using the bathroom. She doesn't walk, run, or play like she normally does, and is up through the night yowling.
That lump, it's been in the back of my mind for weeks. She appeared fine at first, but it kept growing. Now, it's roughly the size of the top half of my thumb. It keeps growing, and I don't know what to do.
I thought maybe at first that it was a lipoma, which is a fatty deposit that shouldn't cause much worry, but with the rate that it's growing, I fear that it's cancer, or a tumor of some sort. 
I can't lose my cat, she's the only one that's always been there for me. 
Through everything that I've experienced in my life, she's been there for. All the friends that I've lost, all the sad times that I went through, when my family had its own issues, she was there for me. And I'm not prepared to lose her.
A physical exam costs around $70, and a cell biopsy costs around $180, but I'm not sure if other costs are necessary. It may go up depending on her current state. I hope that surgery isn't necessary, but with the size that the lump is, it might be. 
I can't afford this, I really can't. I have to pay for my college classes soon, and I can't get a paying job in time that payments are due.
Please don't feel mandated to donate, but please share this around. I can't lose my best friend, I'm not ready.