A great-sounding guitar amp for $30? Gotta be kidding, right? Well, that's what Critter and Guitari claims to deliver with their Box Amplifier Kit, a DIY package that lets you turn any throwaway box (cardboard or otherwise) into a functional amplifier that you can plug real instruments into.
As with any box kit, the set contains everything you will need to slap together a working amplifier. Simply solder the parts as directed and find a container to pack them in (cutting holes for externally-accessible parts, of course). You can stuff it just as easily in a shoe case as in a garbage can, allowing for a wide variety of cool enclosures.
It comes with a 1/4-inch jack for plugging in your instrument cables, along with a speaker and a small set of controls. You will need a 9-volt power supply to crank it up, although it can be modified to work with a 9-volt battery if you need to play on a less controlled environment (e.g. while busking in the streets).
Apart from holding the electronics, the Box Amplifier Kit also serves to shape the sound, which means you can experiment with a variety of containers to arrive at the kind of results you're looking for. If you like to smash your gear after shows (as a homage to all the drugged-up rockstars who used to do the same), this should help cut your costs down considerably.