Starting with the Dash ones...
This one is extremely hard to get since it is the only model which you can switch out his helmet for hair.
These are common on eBay (if you are willing to pay the price). Trigger, Tron, and the Kobuns are harder to find (actually, I never seen an actual picture of the Trigger one until now). I'm pretty sure these aren't made by Bandai, by-the-way. (But I don't feel like shuffling through my action figures right now.)
There were a lot of plastic package "kits" for the Rockman X collection. I want to say they were to "make your own reploid" or extra armor for existing kits.
Despite popularity, Zero isn't too rare. (I was able to get one for $15 on eBay.) Usually when it comes to the X series, actual game armors (like the Falcon or Ultimate) are harder to get than the weapon armors (quite common) or even the "plain" versions of X and Zero.
Now this one is the most coveted model from any series. At first people thought it was a prototype model that wasn't released, but I have seen one or two posted on the web. I've also seen one on eBay a long time ago in the $100 range!
In the original series, the "metal plated" version of Blues is more common than the "plain" version. I haven't seen a Bass or Duo model for a while. The robot masters are common. Although Rush isn't too hard to find, I suggest getting one. I find his model neat and different from the others.
Don't ask me about the EXE models. I never collected them.
So, seeing how rare some may go, the best bet is on eBay. Luck and patience are always key here. But always try your asian markets and toy stores. (Check out your city's yellow pages and online classifieds.) I picked up a few rare ones at dirt prices.
Oh! I forgot to mention, Bandai did re-release some of the X series models. They come with different box art (usually with game art than the actual model), but don't be fooled. They are the same models as before.
Edited by Breakman on April 12, 2009 at 22:22:24.