What's the cheapest way to link a few microcontrollers wirelessly at low speeds over short distances

You pretty much have to buy pre-made modules, you can't expect to wire up your own transmitter/receiver from a few transistors and a crystal, RF circuit design is unforgiving and all but requires a custom PCB (or custom IC) to do. You could probably build your own RF module on a PCB if you did some work, but at that point if you are making your own PCB's, you're not saving much money versus the very cheap modules that are available.

SparkFun has RF Transmitters & Receivers for $4 and $5 respectively. Since they are just basic parts, you will need to do a little extra logic on your microcontroller to compensate for interference, eg sending error control codes so that missing / flipped bits can be detected and recovered.

I found SeeeeeedStudio sells almost the exact same thing, but even cheaper. It's $4.90 for a pair of a receiver and transmitter.

Infrared emitters and receivers can be used to transmit/receive data. However, interference with other devices like remote controls might be a problem. If the connection needs to be really reliable, this probably wouldn't be that great of an option. However, you might be able to find a really uncommon wavelength emitter and receiver to limit the amount of interference.

These are pretty common, and very cheap. Packs of 20 emitters for $1 and packs of 100 receivers for $13 can be found on eBay (not including shipping). Lots of electronics stores have them as well.

The cheapest radio modules I know are the RFM12 modules, they do frequency shift keying in the 433 or 868 MHz band.