I wouldn't recommend spending $100 on monitors. I would save up, and if your on a budget the Adam A7s are a great choice in the future. Ns10s are pretty cheap too, Ive seen some around for $300, but there is a great debate over the usefulness speakers, you would have to read into it. I personally like to use them, not for an entire mix, but for reference of the highs and mids. Still..... they would be better for mixing than Imac speakers. Also, they wont give you a super accurate reading of the lower bass freqs...
In the meantime, you could always use the 1/8" output of your mac into some stereo speakers, anything cheap you can find and has bigger cones (5-8") than your imac so that you can monitor the lower frequencies somewhat
more accurately. Something like this... JVC Ghettoblaster
look its free.... You can spread the speakers and face them towards you so that it forms an equilateral triangle into the listening position. They could be awful speakers for mixing, or they could be decent with a treated room. You never know. Switch back and forth between that and your Imac speakers for reference until you can afford more accurate monitors.
Acoustically treat your room as you save up. Buy some roxull like rcb4t2 mentioned and build your own bass traps and some panels for monitor reflections.