Free or cheap software to boost volume of Macbook?
I've done some research for you, and I didn't seem to find anything except paid applications, and one that is a half solution. I only found these three (paid) options:
Audio Hijack Pro (free trial, $32 for the full version)

Audio Hijack Pro will allow you to boost your sound pretty easily. Upon opening the application, click "System Audio" in the list on the side:


Then at the top, click "Hijack":  Once Audio Hijack Pro is hijacking your audio, go to the Effects tab and where it says "Click here to insert effect", click and under the 4FX Effect, click Gain.


You'll then have two dials to control the gain. Use these dials to boost the volume.

  

In my use of Audio Hijack Pro for boosting volume, this seems like a viable solution.



Boom (free trial, $6.99, on sale right now for $4.99)

Boom lives in your menu bar and has a simple slider to boost the volume.

  

If you click on the  button, you will be presented with an equalizer and numerous other nifty controls:





Hear (free trial, $19.99 for a license)

Hear is a little more complex than the other applications. It is jam packed with features, as you can see from this screenshot:





VLC (free and open source)

I'm not including VLC as a complete solution as it may not work in your specific case. However, it is a half solution. 

You may be wondering why I include VLC as a solution. Here's why: VLC boosts the volume of movie and audio files up to 400%. If you open an audio/video file, adjust the volume slider and it will boost the file's volume: