I have been doing that for a couple of years now, and know others that have been doing this for longer. You can still land collections with Copper keys for dirt cheap, dump the common books for a small - medium sized profit, and then hold the keys/1st apps for further appreciation. As Marvel and DC start to roll out more TV shows/movies that introduce lesser tier heroes and villains there will be further room to grow. It is a fun area to speculate in right now.
Even the pricier books have seen a nice jump over the past couple of years. Just last year you could find NM- or better copies of NM #98 for $75 and ASM #300 for $75 - $100 respectively. Now they are both $150 books in that condition.
I did the same with building small hoards (12 - 20+ copies of each) of BA books in 8.0+ when I got back into collecting in the late 90s, focusing primarily on top Adams books (e.g. Batman #227, 232, 234, 251; GL #76; Tec #400; Avengers #93 + a few others) and lower priced Marvel and DC keys/1st apps (unfortunately, that meant no Hulk #181s or GS X-Men #1s ). This was before they took off and it worked out very well for me. Unfortunately, I sold most/all of them between 2003 - 05 when they made the first big jumps. If I had only held on to the best copies a little longer........