I am a new rat owner and I'm dreading making decisions like this so it's good to see how supportive everyone is and how much information everyone can provide.
Aida, the way I look at it is, you can get the surgery done and there is a chance of her not making it through the surgery or passing not too long afterwards. But if you do the surgery, you're giving her the best chance at survival and living happily and not in pain or slowed down by her tumor. If she does happen to pass (I'm praying that this is not the case), well, you tried everything you could. Her quality of life would've been compromised had you not at least tried the surgery and as others are saying, she probably wouldn't be around for a very long time. You're giving it all you've got and I'm sure Oreo knows that. And I'm sure she'll give it all she's got too.
I hope you can get the finances figured out and my thoughts and prayers are truly with you and Oreo. And hopefully the vet brings that price down a little.