I'm loving reading up on this theme... really appreciate the thoughtful comparisons and predictions presented here. I've yet to really start investing in this theme, and I'm trying to decide where to put my money. Based on what I read here (and see on Amazon and eBay), naturally I have to lean towards 21113 The Cave and 21114 The Farm. I like 21118 The Mine, just because it's the flagship of the first series... but I'm not so sure I want to invest in it at less than 50% off. Many have picked these up at $69 Walmart clearance, and that's probably a safe buy, but I'm not so sure that you wouldn't make more money on that $69 if you bought 4 copies of 21113 instead. It's a tough call. The Cave has been an Amazon top seller forever. In fact, the only thing that makes me nervous about investing in it is that it is so popular... there are so many sets out there already... I'm sure big resellers have stocked up thousands. Aside from being the cheapest (currently around $17), I was surprised to discover today that it also has the lowest cost per brick. $17 / 249 pcs = $0.068 per brick - cheaper than 21116 The Crafting Box (even at current sale prices of $41 / 518 pcs = $0.079 per brick). In fact - if you can live without a Mooshroom, a Skeleton, and a few other special pieces - Buying 2 copies of 21113 will cost you $34, and give you nearly the same piece count as the $41 Crafting Box.
So it's easy to see why The Cave is doing so well. It's harder for me to understand why 21114 The Farm is so popular, and also why 21115 The First Night tends to be a slower seller of the theme. I like The First Night. It has a complete "house" which I think looks great in Lego... so much better than the typical "vignette" designs (a "wall" with figures to pose in front of it). Is it all about the minifigures?