Cheap Low Power Signal Transmission To Smartphone
One-way beaconing is part of the BLE specification, supported by both iOS and Android, and can be done for less than $2 (single quantity) in parts (likely well less than $1 in quantity), specifically an nRF24L01+ clone chip, and an MCU that pre-munges the data buffer to account for the differences between BLE formatting and the nRF24 approach on which it is in part based (Nordic has a key role in defining BLE).  With a little web searching you can find Arduino demonstration code to do this, and re-implement it on a cheaper MCU.

Power consumption is primarily a function of transmission frequency - ie, the less frequently you transmit, the more opportunity you have to charge up a capacitor slowly, and then use that to cover the brief power spike of transmitting.  You'll likely need a small degree of boost conversion as well, but well within reason.

Alternatively, if your range is short and the phones are Android type, you may be able to use NFC.  In theory this can be done with no device side power, though it is easier and more flexible if you have a small power source.