If all you want is the absolute cheapest RFM69 node--and not having to wait for a PCB to arrive from a fab--then this dead-bug design may fit the bill. It uses just an Atmel ATMEGA328P-PU chip ($1), an RFM69HW module ($2.50), some wire, some solder, and a modest amount of pin bending. i.e. total material cost: approximately $3.50.
To build it, follow these steps:
Burn the atmega328p-pu fuse settings to run at 8Mhz. i.e. the system clock is to run from the chip's internal resonator. Personally, I use fuse settings 0xFF 0xDA 0xC2, which also disables BOD, but you can leave BOD enabled if you want.
Burn your bootloader and sketch onto the chip (or just the sketch if you prefer to do it that way).
Make the connections shown in the photos.
Connect to a 2.4 - 3.6v voltage source. I suggest 2 AA batteries. Remember: there is no voltage regulator, so don't go above 3.6v, or you will fry the RFM69.
I've tested it using two AA batteries, and it appears to work surprisingly well.