To add on to what Sandy L said below - it's actually multiples of 18 that are traditional. In Hebrew, letters and numbers are the same (like Roman numerals) and the numbers 1,8 have the same meaning as the word for "life" so it's like saying "to life." I think if you have $36 that is a perfectly lovely gift, and only the most tactless of people would say otherwise. Almost all my bat mitzvah money went into my "college fund," and I think that's a pretty traditional response, so it's not quite the same as tucking $100 into a holiday card, if that helps ease your mind.
Still, if you'd rather do something besides cash, any other "serious" gift would do. A lot of people do savings bonds - you could see if you could get an Israeli savings bond, which shows that you put some thought into it. You could also get a nice leather wallet or monogramed money clip. It is much harder to think of what to get a boy - I know I got a lot of jewelry.
Hope this helps! Have fun.