I'd like to know that too, have been considering doing some zigbee tinkering, and battery powered would be a requirement. I've read in some other comment that nRF would be much better in that regard. Need to do some googling for numbers...
Digikey has the modules for under 4 EUR in unit quantities, but they aren't the friendliest to integrate, since they only have pads on the bottom.
I also found some boards with the bare chip for just over 4 EUR there, you can also find similar ones on AliExpress.