Don't want to worry you but price is directly proportional to the loading of precious metals on the cat monolith.
Most cheap cats have a miserly 5gram loading - in certain cases enough to turn the cel off - however, the durability of the cat will be crucially dependent on the loading.
Nothing worse than getting to you MOT guy and failing due to the PM's having burnt off, and then confronting the manufacturer who will squirm out of it by saying - it only has a one year guarantee, or is the car mapped as this invalidates the warranty or we only allow the warranty if you use our map.
And all the other B--- ---t excuses for ripping you off in the first place.
My advice is save up for a while longer and buy proper kit - trust me the wait will be worth it in the end.
Cheers
Nige