The most valuable catalytic converters are the ones that are the largest. Catalytic converters help reduce the pollution put forth from a car engine. These converters contain precious metals that are very resistant to wear and tear. A normal sized converter coming from an average sized car will sell for $50-$150 depending on the condition it is in. Used converters will sell for a much lower price and are often times not worth removing from a car to sell separately.
There has been a large increase of incidents where people were getting their converters stolen right out from under them. Thieves have been known to go into car dealerships and steal the converters off of brand new cars. There was also an incident of converters being stolen from hospital parking lots in broad daylight. Theft of these converters is due to the fact that they contain amounts of platinum, rhodium, and palladium.