I like to think I have a formula, but the the truth is probably closer to "I buy what I like".
My interests are G1, BW, BM, RID 2001 and Armada, so any CHUG updates of those are welcome. I try to tune out any of the newer series like Animated, Prime and Earthspark.

The one rule I definitely have is that I will buy any Masterpiece or CHUG Diaclone deco they do.