Most of the idle hunting cars were the earlier 9th gen Corollas and some of the 2005+ models with DBW throttles. Wasn't a known issue with the previous two generations of Corolla/Prism cars.
As far as fixing it, on the cheap, and not dump too much money into a car that will probably be replaced soon? Hard to say, you'll pretty much covered the "easy/inexpensive" fixes. Now, it sounds like it could be anything from a bad chassis ground (electrical noise will confuse sensors and ECM), bad IAC circuit, vacuum leak, any number of sensor issues, timing belt could have slipped a tooth on you, making the car run odd (if you have a timing gun, give it a quick check, if I remember correctly - you'll have to jump the diagnostic port under the hood to get the ECM out of the loop).
Since it happens when the car warms up - I would suspect O2 sensors as another likely suspect. As the gets up to operating temperature, the ECM will use the O2 sensor output for part of its idle control feedback. Vacuum leak is also likely, bad IAC valve in the throttle body is also a good culprit. Unfortunately, those are also pricey parts. All combined, you'll probably spend more for parts than what the car is ultimately worth.
Another thing to try, especially if the commutes are that short (~3 miles). I'd take the car out onto the highway and just run the snot out of it. Could be a simple case of heavy carbon deposits inside the combustion chambers. If a car hasn't been run hard from time to time, it can generate a surprising amount of carbon in there, and not have a good way to blow it out.