Designing a low power wireless temperature transmitter
For lowest cost I would recommend a simple ISM transmitter (frequency will depend on where you are (which freq is allowed)). A quick search on digikey.com will point you in the right direction.

Some more detail needed:

Is data going to be one way always?
Is there more than 1 transmitter per receiver? 

What is the required battery life?

This is type of application where energy harvesting is great, perhaps research this and include a rechargeable battery - although this will increase the board cost significantly.