If I'm entering a place that has food of any kind I'll just ask what they want, I've rarely been asked for something that exceeds $5-10. More often than not they eat, pack up their stuff and leave after that. They feel acknowledged, I feel good for helping and I also hopefully didn't contribute to the drug issue.
Don't let perfect be the enemy of good. You're on a budget and doing what you can. Don't sweat it.