I would agree with this approach as well. While my sample size after getting back into collecting is much smaller (mostly limited to high grade SA Avengers), I'm of a similar mindset. The last few years, I've split my collecting funds into a monthly budget for the more common non-keys while stockpiling a rainy day fund for the keys like #1, 4, 57 and the rarer books in high grade like #3.
I'll second the notion that the filler books tend to hang around, but when a key pops up at the right price--you jump on it if you can.