I don't entirely follow your logic that she is acting entitled to your money. She is worried about your mother having information about your giving your GF money and may generally feel uneasy about an adult who shares his account with a parent. It's an odd thing to care a lot about, for sure, but doesn't tell me she's a spoiled gold-digger.

She seems a tad emotional but if she's a full-time software developer and in law school, so maybe she really needs a break and your support in that. She's trying to make the puzzle pieces fit for a getaway, in a way that seems pretty light in demand on you, and you're waffling. Why don't you find a trip that you would be comfortable with to offer as an alternative?

Also would the $1500 trip be shared between you? If so, for the 'base cost' you're only in it for $750, which seems like a reasonable amount to spend if you have a well-paying job and you wouldn't have to leave your contract early to take the trip. Regarding the recommendation for how much to budget for spending money, I don't really think you should use that as a barrier. You can spend far less (or far more) by carefully choosing what you do, how much you gamble, looking for less expensive eateries or getting the buffet package, finding online discounts for shows, etc. You could probably have reached some agreement with her for how much you could each budget for spending money so you could pool it and have a good time.

Overall, I suggest you go easy with her on the trip and work with her to find something that you can pull off. If you don't want to make adjustments to your account, tell her you don't see the need to and it doesn't impact her, so you want her to let it go.