What does each category stand for (moderate, poor, depleted, etc)? Specifically, I'm looking how much resources exist in each category to help determine if it is worth probing a planet down to depleted.
Depleted planets may still contain spikes of resources, so the decision to mine any given planet is determined by how long you are willing to spend scanning. Obviously the rich planets will yield the most amount of resources (about 10,000 units) but you will be lucky to get a single spike of 2,500 resources from a Depleted or Poor planet. In general, I scan the Rich and Moderate planets all the way down to Depleted (unless I have reached my target).
I think the most important point in getting the most resources from your probes is the size of the spike. It seems to follow a logarithmic scale. Spikes that hit the 6-7th bar will give you twice as much resources as spikes in the 4-5th.
When you scan a planet, you get an initial estimate of it's mineral resources. This can range from "Very Rich" to "depleted". While you can "deplete" a planet's resources, the "depletion level" for the planet is only a general indicator of how many probes you've launched at a planet and total remaining minerals.
Even planets which show up as "depleted" can be lucrative to probe, as long as you stick to medium-high spikes on the mineral scanner.
It has nothing to do with how many minerals you get per probe launch, as that is solely determined by the height of the indicator spikes itself.