If it's your girlfriend, you should know her personality and what she likes and dislikes. You supposed to know her. Given that you have $50 budget for valentine's day, if you know your girlfriend is the type of person who likes receiving gift, you could've been thoughtful on what to give her. It's not about the amount, it's what will make her happy. With $50 you can give her something useful as well while adding effort, like a simple letter.

Yes, money is useful. But for valentine's day, it's kinda normal for women to expect something aside from money. Because thinking of something to give to your partner is also part of the effort which makes them feel loved. You can't say that a person lacks financial sensibility just because she expected a "gift" during valentine's and not money. It's a special day to celebrate love so like, at least show some effort... But there are people who appreciates money more rather than stuff with no uses. So you should really know what your partner likes. Consider the thought of the person you'll be giving a gift.