$20 for POE to USB alone is quite ridiculous markup to count on top of each sensor. It will be cheaper to connect 230V cables everywhere (I’m still at the design phase for the house) and just attach USB chargers everywhere.
You can buy battery delete kits or just run wires to every device. The #1 thing you need to pay attention to is the voltage of the device. If you apply too much voltage you will cook the device. Here’s a picture of me powering a z-wave device that takes coin cell batteries with a power supply.