They're out there; people just can't be bothered to put it up on the block for a little bit of cash. A key to collecting is being patient. Pretty much almost anything comes up on the block eventually. When you find your prized item after a long search, it'll be more than worth the wait. Good luck!
As for collecting common pieces vs. rare pieces, I say just collect what interests you. I personally like the rarer pieces because of it's charm and uniqueness. I'd take that over a common black frame and grey lenses any day. That's just my preference. If someone prefers Black/Grey, I say go for it. You're the one who's gotta be proud of your collection in the end.
They were everywhere, and now, as far as I know, I'm the only person here that has them. By that rationale, they're worth more than an OTT any day of the week.
An item is worth only what someone else will pay for it. You can have a true one-of-a-kind item but if no one wants it and are not willing to pay for it, it'll be worth less than something that may have a million pieces made but may have a million-and-one collectors going after it.