Tips on parts for a very low power LED blinker
The six-pin SOT-23 PIC10F200 is $0.32 for 1ku, and can use a supply down to 2V. That would be a good choice. If you need PWM it could be implemented in software, as the device includes a timer.

Another option would be the similar ATtiny4 from Atmel. It has more features, including PWM, but is more expensive at over $0.50 for 1ku.