You should be offended even if she takes $5 without asking, but $500 is a significant breach of trust. Confront her immediately and ask why she would do such a thing. End the relationship if she continues to lie or make excuses.
Confront your girlfriend about the stealing, point out each incident, and make it clear you know. If she lies, that's a sign to end things. Don't stay with someone who is dishonest; it's not worth the risk of dealing with more severe thefts later.
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