Jimster:
The range will be 30ft max. I'm looking for the cheapest option.
Functionally, the nRF24L01+ radio transceivers are just what you need.
They're tiny little cards that can be wired directly to any Arduino and take almost no power. Each instrument would consist of a minimal Arduino and the radio transceiver, plus power supply. The low power also translates to low range. I'm not sure the cheap non-amplified versions would stretch to 30' under ideal conditions, and if there are any obstructions I'd say that was out of the question. There is an amplified version with increased range but I have no experience of them.
If you take this approach and find you're running out of range, you have the option of installing additional units to act as relays.