You should also consider the growth in the future interest of Lego (Market Potential). This is all speculation, and that is all it will every be. The more Lego expands and creates new markets, the larger the collector base will be. I keep thinking about an article I read a month ago about a family who spent $100,000 on Beanie Babies, and ended up with nothing but crappy stuffed animals. They actually invested the money from their children's college fund, hoping to double or triple it. I think about that every time I buy a set for investment.
Lego has a few major differences, however: 1. We are able to purchase sets as deep discounts, if you know where and how to find them. 2. The bricks themselves have actual physical value. People will always play and use Lego blocks, as something doesn't come along and take its place. Looking at the Lego knockoffs, I don't think that is every going to happen. 3. Lego sets are not priced to sell as collectibles. We can watch for a product and purchase it at normal retail price or below. I do not think any Target, Walmart, or Lego.com will every sell an item at an increase because it is a 'collectable'. We might not be able to get every set or LE set, but those sets will enter the market at a regular retail price. 4. And this is the big one for me: I love playing with Lego sets, and I am 42. My son and daughter love playing with Lego sets, and they are 2 and 10.
Lastly, as a former stock market trader (Options), I see the risk inherent in purchasing something and losing money, maybe even everything. To say something is a sure thing and will 100% assuredly make money is the fastest way to going broke. That being said, let me add that I believe the worst vice is advice, and I never give it unless asked. My reasons for collecting and investing in these products are my own, and I question them every time I purchase a set.
Right now I only have 20+ sets set aside for investments, so my risk right now is negligible. If I do decide to amass 100+ sets for investment purposes, there will be much research and decision making hours taking up a portion of my time. Right now, I enjoy it because I like Lego sets and I find the whole market interesting and something I want to invest both my time and money to.
Here are some quick reads about Market Potential. I see some usefulness in the analogous products, and it is an interesting concept to add to your buying/selling habits.
About plants, but has a nice equation:
http://plantsforhumanhealth.ncsu.edu/extension/marketready/pdfs-ppt/business_development_files/PDF/estimating_market_potential.pdf
Bottom up and top down:
http://mpd.me/addressable-m-1/
Market size hypothesis:
http://ultralightstartups.com/wp-content/uploads/2012/04/Steve-Blank-Market-Sizing.pdf
Another calculation:
http://www.marsdd.com/articles/estimating-the-market-size/
Good luck, and thanks for posting your ideas. They made me think a lot more about choices I will have to eventually make.