The all-important left turn signal
"I'm taking the lane"
Bicycles usually go around parked cars, not slam into them.
This is the only way to get the message across, it seems...
Anyway, IIRC these lights cost about $1.67 each . The OP asked about the cost of the light,, not the cost of the battery, which is 12 volt. I recently converted from. Lead/acid to Lithium Iron Phosphate, but the cost of LiFePO4 has come down to where it is more economical than lead acid.