Digikey has a pretty good web site for search, start with ARM M0+ and then select for what you want. I picked the 3x3MM QFN package as a selector and got a number of Kinesis parts, here is one that in 100 unit quantities is .50 each. In 10K I'm sure that is under 40 cents.
http://www.digikey.com/product-detail/en/MKL03Z8VFG4/MKL03Z8...
This particular part 8K flash and 2K RAM. Now the M0+ is better for these low end parts because it has the Thumb instruction set which gives it better code density than the original M0
