Well, if you're a member of the RPF or Rebelscum, I would definitely recommend checking out those sites first because you can find some great buys there and save a LOT of money when compared to Ebay or other venues.
It also depends on your focus. Do you like Boba more than other characters? There's a guy on the RPF that is doing a McQuarrie Concept Boba Helmet that looked pretty awesome.
My particular collection focuses around Vader. another individual is doing an AWESOME McQuarrie concept Vader for $550 right now. Stone is currently taking an interest thread over there. I'm not sure if he's the one who also did the Boba.
You can usually find a bunch of stormies for sale, including the blackhole/black stealth trooper helmet.
Really it comes down to what you're interested in. You're the one that will be admiring it every day as part of your collection, so make sure it's something you want. I can say every one of my Vader helmets, sabers, and other memorbilia has a special place in my heart and will be something to pass on to my sons someday.