Arduino pro mini 3.3 V Version!  1,57 €
    --  alternative 328P: 1,36 €
RFM 95 W 4,72 €
Ublox NEO-6M GPS Module GY-GPS6MV2 NEO6MV2 6.19 €
    --  alternative ?  2,16 € 
Ein / Ausschalter  0,08 € 
Battery LiPo 3.7 V 1200 mAh 5,30 €
    --  alternative: 4,61 €
3D enclosure (Thingiverse)


    total costs as of today, cheapest variant: 1,36 + 4,72 + 2,16 + 0,08 + 4,61 =  12,93 €
    and some cables and board

Antenna for base station:
https://de.aliexpress.com/store/product/Glass-Fiber-Antenna-Gain-8dbm-Transmission-range-is-further-LoRa-Gateway-Antenna-with-433-470-868/2805180_32852314761.html