I believe the OEM radiator is somewhere north of $400, which is why many people opt for the Koyo or another aftermarket rad. With mine, I replaced the upper and lower radiator hoses and clamps, the thermostat (I replaced this twice actually) and the 3 nuts that secure the thermostat in it's housing. Probably around $210 not including the coolant and distilled water.
I purchased my OEM radiator from olathe toyota parts online, and it cost $216.99. I bought it with a bunch of other parts, and shipping was $34.
For the extra money, I think the OEM is worth it for the correct fitment. Not to mention my other one lasted 150K miles.
As far as the external cooler, you just have to judge if the cost is worth it to you. I would be inclined to say a radiator should be replaced every 150k miles anyway.