Before you go on to buy any computer speaker I would recommend that you listen to the speaker system playing music. As it is not possible to go on to a store and play music on each of the music system so the best place for this is watching videos online. Here is an excellent hubpage about the computer speakers having a lot of videos and sound tests.
http/markmorgangenius.hubpages.com/hub/21-best-computer-speakers-under-100
First decide whether you need to buy a 2.0 or a 2.1 computer speaker. The 2.0 contains 2 speakers and a 2.1 speaker system contains 3 units(2 speakers and a sub woofer). After that you need to listen to the sound. Watch the videos in the above hubpage to see what type of speaker you need to buy. I would recommend that the music system should have a solid base as it sounds better than small shrill speaker.
If you are an audiophile and need that perfect thing that plays all the highs and lows extremely well then I would recommend you Bose series III companion II speaker(these are 2.0 speakers). See their sound test in the above hubpage and you will come to know what quality of sound I am talking about. These are on the bit expensive side $99 but falls within your budget.
If you want to buy a 2.1 speaker system then I would definitely recommend Cyber Acoustics satellite system(Model No CA-3602a). I own them and I love these speakers. Have a glimpse at the sound test in the above hubpage. They are extremely affordable($40). I differ from many people here especially those who run after the brand name blindly. Do not just go after the brand name rather see the quality of the product. These speakers will rock your house and are the perfect thing for a party. They are not just loud but also have very crystal clear sound.
If you are much inclined towards gaming then nothing beats Genius SW-G2.1 1250(It is a 2.1 system). They have quite a different look with a red and blue color options of the speaker with a black casing. Watch the sound test in the above hubpage. They would cost you around $50 from amazon(everywhere else the price for them is too high)
I would recommend buying from amazon as they have a huge discounts on various speakers so its wise to save some bucks plus everyone knows their service is the best.
Recommended webpage:
http/markmorgangenius.hubpages.com/hub/21-best-computer-speakers-under-100
I hope that helps!!