Cheapest option for split screen video editing?
The cheapest option is to use quartz composer to assemble the individual clips and mix them down. It is not designed like iMovie or Final Cut Express with shortcuts and ready build transitions, but it is free with Xcode 3. You have to register and agree to Apple's developer tools restrictions before downloading Xcode 3. You want the free program at the bottom of the page.

A more expensive solution would be something like ScreenFlow where you could make several self contained movies and then play them back for recording on the screen. By arranging and synchronizing multiple quicktime windows on your desktop you could record what you see on your screen. I have seen amazing works that were recorded as a screen capture. It's also not ideal, but if you are good, you can do wonders. Quicktime has free screen recording as well, but ScreenFlow makes it easier to crop out parts of the screen, catch the audio and automate moves and edit.

Lastly, Final Cut Express is the best tool and it sells for under $200 US.  It is the same engine as Final Cut Pro, but lacks all the other tools like Motion and Color that assist in cinema quality effects and editing.

Only you can know your budget and how much your time is worth. Start free and only pay for software when your time is worth more to you than the cost of the tools.