What wireless temperature monitoring systems should I look into?
One option is using Arduino based microcontroller. I recommend JeeNode for integrated wireless.

For basic one "server" plus one remote sensor you need two JeeNodes, USB adapter, temperature sensor and battery, all for around 55 €. You need to know how to solder electronic components and program in C. Each additional sensor would cost about 20 €.

Here is example project for temperature sensor using Arduino (will work on JeeNode as well). You have to add wireless code as well, starting with this skeleton.

So, not so cheap and certainly pretty advanced DIY. ;-)