I have two cats and I can tell you, they are not that expensive. Afford yourself one big bag of cat food, you will probably be able to find a big one that will last long enough.
Also, cat can eat almost everything you do. Even more. For example, when my family eatch chicken, cat eats bones lol. She literally eats bones, cracking them with her teeth. She also needs water and company, and that's about it. Train her to do the messy thing outside and you should not have big problems.
One thing you have to take care about though is baby cats

I don't know about the prices where you live, but here it costs around $50 castrate cat. You will also probably need vaccine, that's around $15 here.
So yeah, in the beginning, you will have some expenses, but I believe you can manage somehow, cat is really cute, she could get donations very easily
Good luck!