A wedding gift for another person shouldn't cause a hardship, so don't spend more than you can afford obviously.
Typically, good friends get a gift around $50-$75... but this is if you have it just lying around.
Just remember, giving a wedding gift is completely optional. The fact that you are going to her wedding should be all that's expected... (in a perfect world
)
I would go in on something with someone else if you can.