I've got a Belkin SOHO 2-port KVM, switches Keyboard, Video (using 1280x1024, and iirc it can handle up to 180khz), Mouse, Speaker and Mic, there are buttons on the front for switching, or you can use keyboard control (scroll lock twice, then either up/down, the number of the machine, or something else which actually puts it on a "security camera" style flicking every few seconds)... :-)
On the downside, I think it cost closer to £100 than £30, but that was new, and as you can also get 4-port ones like that you might be able to get a second hand one somewhere.
I did used to use a 2-port manual switcher (still got it somewhere) - but it only did one ps/2 and the monitor, and depending on how the pc behaved it could sometimes lose all connection to the ps/2 device and require a reboot, the belkin one is nice in that it "fakes" it when the computer isn't actually connected to anything, so i've never had it lose it ;-)
Robin