It depends on what type of filming you are planning to do. If it has much movement in it, you will need some sort of boom pole and shotgun mic. If you are just doing interviews, you can often get away with a digital audio recorder. The prior is going to cost you more than $100 for anything that is worth the money being spent. The latter can be accomplished passably with the on-board mic of an H1, which can be found new for ~$100 or used for ~$70.
Obviously $100 is not ideal. Even in the best of situations, the H1 is still going to lack the quality of higher priced mics. That being said, if you are currently working with the camera mic, almost anything will be a vast improvement.