It's not a selfish post at all; just an honest question! The fact that she has already asked what you wanted means that the topic is open for discussion, and you can feel free to bring it up simply by relating it back to the initial conversation, e.g.
Hey, remember when you were asking about what I wanted for my birthday? Well I have been thinking about it and...
I am unfamiliar with the specific details of your relationship, but open and honest conversations about what is or is not financially doable, approached with realistic expectations, are important if not always the most comfortable. This may actually be a good way to "test the waters" and segue into talking about such things. You can try asking for this item in a way that conveys it as more of a "wish-list" gift without directly mentioning money, and have something else that's less expensive in mind just in case it seems like she might be uncomfortable with the idea. For example some version of,
I have had my eye on 'x' for a while now; does that sound alright? It's totally fine if not, there's this other thing I was looking at too.
If you are less committed to the item in question you can also give her a few things you are interested in to choose one from, and she may organically choose the one that is around the ceiling of what she feels she can afford.
I hope this is helpful, and though I have already mentioned it I highly encourage putting yourself out there into having more open discussions about money. It's not at all romantic, but as your life together progresses having the topic available and not set aside as taboo could help alleviate potential pressure points. Good luck, and happy birthday!
Citation: I have personally experienced similar situations in relationships before wherein the topic of money, especially in the context of people who make different salaries, can make open conversation and planning about gifts, travel, meals, and the like more difficult. Such situations have always been best resolved through transparent and practical conversation.