I remember seeing a previous article on HN talking about how you should have 3 products to sell instead of 1, because it helps to anchor the customer when they make a decision. That makes a lot of sense when you consider the study shown in the article about how people valued two books in isolation and then relative to each other.

One thing to possibly take away is that if you have a more "featureful" product (like the 20,000 entries v 10,000 entries) that's not as nice, people will be willing to pay more for it, if it is next to a nicer "less featureful" product. Obviously, this might only work in limited domains, like the dictionary example, but food for thought if you're selling your own product or one on eBay. And whether people would actually end up buying the 20,000 word dictionary instead of the 10,000 word dictionary is another question entirely.