urge you to look here:
There's bound to be something within your budget... but don't expect really great sound for $100. In speakers, you do get what you pay for.
Speaker design is a very complex science as well as art.
What limits the power input mainly is the size and impedance of the voice coils, and the design of the crossover. Small voice coils handle a lot less power than large ones, primarily due to poor heat dissipation. If you want, you can even make a basic crossover with just one sandbar resistor and one electrolytic capacitor per speaker to save money. It won't be as precise as a more complex one, but you have to keep your budget in mind and think of tradeoffs.
I've been involved with home stereo equipment for over thirty years, and this site is very highly spoken of in the audio forums I haunt. Good luck!