Depends on the mic. But it's not "boom mic", it's "shotgun mic" that will suit you best. No such thing as "boom mic". The boom is the long stick that you attach the mic to, to get it closer to your subjects.
Super-cheap shotgun mic's can be found for around $30-$50. Needless to say, they do not sound particularly good, but using them can be better than using the on-cam mic. This kind of mic is unbalanced, meaning it's the same kind of plug you use for headphones.
A good mic starts at $300, and that is really the low-end. They will require an XLR input, which are only available on high-end multi-thousand-dollar prosumer cameras. To use this kind of mic on a lower-end camera, you'll need either an XLR-adapter (works well for low-end prosumer cameras) or an external recorder (necessary for DSLR, also works well for prosumer cameras).