Hello, you should give a try to "EasyPCB" it's very convenient to use and to make first sensors that are clean and without wires everywhere. It keeps things simple but still have a connector for each pin of the pro mini. I found it really great when I started with MySensors and still use sensors made with it now.
Cost will be very low:
less than 1.5$ for the PCB
less than 2$ for the arduino pro mini clone
less than 1$ for the NRF24L01+
Less than 5$ including capacitor for the radio, resistors, solder, ...
5$ left for the sensor, that's enough for a lot of options.
https://forum.mysensors.org/topic/2740/easy-newbie-pcb-for-mysensors/
https://www.openhardware.io/view/4/EasyNewbie-PCB-for-MySensors