Recently Spielbox found some older copies in their basement, so a lot of us were able to get "The Plague" and "The Tunnels". Cundco had a few Besiegers left, but they sold out very quickly. I wasn't able to get one in time, but luckily a member here has been nice enough to ship one to me.
The real rarity is "The Cathers" which is similar to besiegers, but older, rarer, and more costly.
I assume "Little Buildings" and "The Meeple Stickers" are cheaper because they aren't nearly as interesting as what is listed above. I haven't checked in awhile, but last time I did you could still order the Spielbox with those promos. The Plague used to be very expensive on Ebay, but I imagine it has gone down in price because of the recent Spielbox release.
Also, not to make you spend any more of your hard earned money, but if you can find "The Ferries" or "The Flyers", they are worth your time.