Investment in 1-character short domains proved to be a good opportunity. Analysing the 1-single-letter-domains.com sales, that belong to the lowest price sales segment for this assets (so called fast-flips) the average domain value grew by 238% from the year 2013. The strongest growth by value can be seen 2016. This trend shown in the providers own data seem also to be backed by other numbers like auction sales on Flippa where many domains get sold currently at prices around 2500$.
The numbers are described in this article: http://1-single-letter-domains.com/news/news2/