catalytic converters generally run anywhere from $100 to $1000 depending on model of vehicel. Brand isn't always that big of a deal unless you are doing something high performance.
If you are finding Cats on ebay for 25 dollars, chances are high that they have been hollowed out. People do that for better performance or loud exhaust note.
The reason they are so expensive is because they are made with platinum inside. Part of of the internal make up with porcelain to reduce emmisions. thats why you will find people stealing other peoples exhaust or buying used and junk exhaust.
Depending on how nice your car is and what state you live in, most times you can forgo the cat conv. with staight pipe and muffler, and your car won't be obnoxiously loud. Most states its illegal to not have a cat though.
Be weary of cheap cats, repair facilities as a whole wont knowingly install something illegal on a road going vehicle.
