The real cost is the additional damages to the local ecosystems, which then spreads further afield to the pollution of air, water and soil, contributing to climate change in terms of :
oil extraction, manufacturing, transportation
waste collection and treatment (landfill, etc.)
effort required to remove it from the environment and then dispose of it
if it is not removed, the plastic bag will degrade further in a river bed or in the ocean, possibly killing mammals or birds or entering the food chain
The real cost is the additional damages to the local ecosystems, which then spreads further afield to the pollution of air, water and soil, contributing to climate change in terms of:
oil extraction, manufacturing, transportation
waste collection and treatment (landfill, etc.)
effort required to remove it from the environment and then dispose of it
if it is not removed, the plastic bag will degrade further in a river bed or in the ocean, possibly killing mammals or birds or entering the food chain