If your BF or GF has a reasonable outlook on life, they wouldn't even allow you to spend month's salary on a gift for them, they would make an effort in saving for that thing themselves. However, I don't see anything wrong if you were to save over a longer period of time to be able to get what she/he wants to make the person you love happy with it.
How much would I spend? Right now, absolutely zero. Don't get me wrong as I used to be very much into buying things for my wife. I think it's great that people take the time to consider the wants and needs of their GF/BF over themselves.
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