Ah well.
If you know someone nearby who's handy with a soldering iron, you could knock it together for a fiver (inc case and connectors). The number of components needed really is small, the opamps plus a couple of resistors.
It doesn't quite qualify as a passive mixer (requiring a power supply) but it works.
You could just get some audio connectors and connect them all up in parallel. I wouldn't reccomend this however - especially when the different machine outputs may have different impedances etc...