What you are seeing here at the center of the shot is a RR-Cirkits BOD-8 board. This is a board made to detect occupancy of up to 8 blocks, using current transformer coils that get placed on the DCC feeder for a rail of your choice. Also in the picture - Tortoise switch machine and Digitrax UP5 panel.
You can see that I use red for DCC Rail A and Black for DCC Rail B as the DCC bus and use 3M suitcase connectors to go from bus to feeder gang to feeder.
Per detected block, this is a probably the cheapest commercial way to do this. $35.40 is their direct NMRA pricing so that's $4.425 per block. A Digitrax BDL168 (which I used a bunch of on the previous railroad) runs about $120 for 16 blocks, which makes the math $7.50 a block there.
Once you figure out how many blocks even a smallish railroad requires, you try to figure out ways to maximize your ROI...