All cats are expensive for several reasons, the precious metals,
platinum, palladium, rhodium, cesium and there's a couple of new ones too. Look up the price per ounce of these. Some OEM cats can carry an ounce of each or more.
There's only 3 catalyst manufacturers worldwide. They all (oem and aftermarket) get their beds and cat bodies from pretty much the same place. Price is only determined by the amount of metals within and size of beds.
Aftermarket cats, by design are not required to be as efficient as oem, usually 30% for NOx and 70% for HC and CO, vs 98% NOx and 99% for HC and CO. This is why many will almost immediately code efficiency. They don't have the oxygen storage capacity or reduction and conversion abilities.
They don't have to have the same warranty, and the connecting pipes don't have to be corrosion resistant as the 409 stainless oem cats are, only the shells do.
Many older cars have less strict cat efficiency monitors than newer cars do.
Here's examples:
1997 Lincoln Continental, cats no good, get a bs labeled OBD2 cat from an aftermarket exhaust company for $600 (is a Y pipe with 2 cats), flunks 1 bank within a week. The next 3 from the same company do the same thing. OEM is obsolete, and listed at the time for 3500, the CA CARB certified cat is avail to be shipped from CA for almost $4000! Car is junk, as it can't pass any emissions test in the Northeast.
2003 Ford Econoline Van 5.4 V8, driver ran it on misfire bad enough to melt 1 side primary cat (Y pipe) and send melted junk into primary large under body cat. Price of Y pipe, $1200, price of very large underbody cat, $3150.
Any Ford Escape V6, close coupled cats at each bank. These suffer from coil failure and misfires. Melts usually both front cats, sending pieces into rear cat destroying it. 3 cats are just under $3000.
Toyota Tacoma pickup, has 2 under body cats, both are monitored via sensor after second cat. Both cats list for $2400.
Toyota Sienna V6 mini van, has 3 cats, front 2 are monitored, Toyota has updated them, you can replace 1 bank at a time, each lists at $1400.
On the OP's car, how much oil does it consume? I'd want to see fuel trim values at idle, 2500 no load, and cruise at 45 light load. Also, any exhaust leaks, even the tiniest can cause a cat code. Rarely a poor performing rear sensor (monitor) that doesn't code can cause false P0420.
