If lucrative, I would like to get in on it (duh). I'm not going to build a rig or anything. I want to use a USB device only.
Most ASIC miners, including "rigs", are USB devices in the sense that you connect them via USB to a pc that controls the miner. By your post I assume you mean USB devices such as the USB Block Erupter (USB-stick sized devices).
I understand it will be slower than a "rig". But what about loading it with an ASIC chip (remember I'm a big n00b here)?
USB miners such as the Block Erupter are exactly that: an ASIC chip on a USB stick.format:
1. Is mining on ANY USB device truly lucrative?
The small devices? No.
2. Is the difficulty THAT bad?
Depends on your definition of bad, but for a smallscale miner that doesn't have hardware yet, it's bad.
3. Loaded with the best chip, how long would it take me to mine 1 bitcoin?
With the increase in difficulty, I'm not sure if these things will ever accumulate an entire Bitcoin.
4. Finally, how much would the "best of the best" USB miners go for?
Too much.