My research for my boxing card book indicates that card markets tend to lag the stock market by about a year. Specifically, the market for boxing cards stayed reasonable until about mid-2009, when it started dropping, and it dropped until fall 2010, when it bottomed out. The rebound similarly has lagged the market. That said, I think the better correlation as to all but the really expensive cards is the labor market and possibly the real estate market. When employment is strong and real estate is rising--as is the case over the last year or two--cards do well. Classic lower-end luxury goods.
One strategy to consider if you don't want to spend big on cards: modern. There are some beautiful sets from 1984 on from which you can get HOFers for less than a cup of coffee. And also the Topps heritage issues and tribute issues. They might not increase in value but you can collect and enjoy them guilt-free.
Another one: 1970s. I'm a kid of the 1970s, so my heart is with the good old Topps issues from the era. You can collect all but the top RCs on the very cheap, especially if you are good with nice looking 5-7 level cards. Even 8-9 from the late 1970s are really cheap right now.
