The problem is that in your question you do not specify things such as income levels or other measures of wealth for either your family or your friend's, which would impact the answer.

For example, if both families earn $100,000/year, a graduation gift of $100 might be fine, but if both families earn 5 times as much, that would seem like a very small gift. The best way to figure something like this out is to scale from amounts you would typically spend on other things, as that would be related to how well off you are.

Thus, perhaps spending as much as you'd typically spend on a dinner out for your family might be a reasonable level for such a gift. Obviously there's a range, as you might sometimes go out to truly extravagant restaurants, and at other times you'd go out to more budget-friendly places. Use that range as a guideline, and then figure out what would be a comfortable point in that range.

Keep in mind also that if your income is much higher than your friend's, you might be placing her in an uncomfortable situation if your gift is too expensive, where she'll feel obligated to give your family such gifts in return. In such a situation it's best to reduce the amount a bit, but still be generous and keep it slightly higher than you'd expect from her if your positions were reversed.

Are you comfortable enough to have a quick heart-to-heart? I did with my grad, with family and friends who wanted to know how much - the same question you're asking. Since you are that close, you'll probably be giving a "family" sized gift as opposed to the "friend of the family" sized gift.

Everywhere I look, the answers are all over the map, but $200 seems to crop up more frequently for money gifts. Primarily, it's recommended that you buy an item as opposed to money, but I disagree. My grad lumped all the cash and bought a laptop for college.

It's like any gift - money never goes wrong. However, you could buy a fetching pair of bookends....