How to prototype with Jennic JN5148/001M00T Modules
Since this question was asked, the situation has changed a little: as Jennic is now fully absorbed by NXP, which also acquired Freescale, there are a couple more options to choose between.

Sticking with the JN516x family, you can now find some pluggable modules with header pins, such as:


DORJI JN516X JN5168 Zigbee module DZM5168M
MeshBee


As an alternative, there is the formerly Freescale Kinetis KW21Z with an ARM Cortex M0+ and an 802.15.4 transceiver with an official development kit (FRDM-KW41Z) made of two boards, for just above 100$. Honestly I couldn't find anything any cheaper than that in a quick search on the web.