ESP8266 Low cost wifi enabled microprocessor around $5
Kootek GY-521 MPU-6050 Accelerometer and gyro sensor package around $8
2400mAh AA size lithium ion battery under $5
MAX1701 power supply chip. will need to build a pcb for this but is efficient at running running full range of lithium ion battery voltages to the 3.3 needed by the esp8266
Soda bottle preform should be available for under $1.
a fishing weight or lead pellets should be very inexpensive.
Even without any savigns from larger buys the product should be under $30 for materials.