The Audio Technica ATR 6550 is a shotgun mic you can get for around $65 US on Amazon. That said, it's not a great mic, but then you're not going to find a great mic for under $100. It will, however, get you usable sound if you're on an extreme low budget.
Here's a review:
https://www.youtube.com/watch?v=oXTbnksUf7I