As I recall, there are two plastic panels under there. One is the large plastic piece that "bolts" up to the bottom of the front fascia and the fender panels and tranny area. This larger one will also have vent channels to cool the brakes on SRT models. The other is a smaller piece that covers the oil area. The larger one isn't removed during oil changes, just the smaller one.
I have a part number for a new larger underbody shield (at home on a quote from dealer) and the cheapest I can find it online is about $130. The regular R/T panel (without vents) is only like $60. You gotta pay a premium for the SRT! I don't know about the smaller piece of plastic though.