Jesus, this isn't difficult.
Question.
"Hey, do you mind if we split this tonight? I don't have a lot of money right now."
Answer 1.
If answer is "No, thats fine!"
Say "Thanks

i wish i had more money to buy stuff, but being a student is tough financially. You're great."
Answer 2.
If answer is "No."
Say "How come? I've not minded paying the last few times, but i can't afford to do that every time if i'm honest. Being a student is tough financially. Is that a problem?"
At this point any reasonable rational person will apologize and won't mind, so go back to Answer 1.
If she does have a problem with this explain you'll have to go to cheaper places, do things that don't cost money sometimes, or just flat out say thats not right.
If she has a hissy fit or gives you silent treatment or whatever shes obviously a spoiled brat and fuck that bullshit.
If you can't afford it, you can't afford it. Theres no point skirting around the issue because you don't want to sound poor. It might be uncomfortable for a few minutes, but if she just doesn't realize what the deal is then it shouldn't be a problem. Maybe she thinks you have more money than you actually do. If she just a bitch doesn't care, then she's not worth it anyway so who cares if its uncomfortable?