There's no rule in picking a vocal mic, you should pick one that sounds good with your voice within acceptable limits.
The classic choice for cheap dynamic vocal mics is Shure SM-58 (some people prefer using the SM-57 finding it more suited to their voice, but Shure sells it as an instrument mic) or an imitation. The SM-58 has a strong 'presence' effect when used at extremely close range and is generally well suited to a whole range of voices, thus has become a standard in concerts.
A dynamic mic is good if you have to hold the mic in your hand, otherwise you could also look for a vocal suited condenser microphone. If you don't have a phantom powered audio card, mixer or DI box, electret condenser microphones are available and are battery powered.
So overall it depends on your needs, and cash. For instance Neumann, Schoepps and high and mid range AKGs sell for a lot more than 100$.
A good starting page (although it doesn't cover a lot of important mic stuff, such as polar patterns - generally cardiod for vocal mics - and frequency response curves) is
here. It's generic, but I agree with the suggestions.