[As you know I am putting together a 1970 Topps set in VG-EX condition. I was able to more or less cherry pick up via trade/purchase 3/4 of the set. It has been inexpensive and incredibly fun. As I said, all depends on the set, and the condition. If I cared about cards being EX-MT to NM I could never have picked up my Munson RC for $10!!
To me this doesn't answer the question as to whether or not it would have been cheaper to pick up a VGEX or worse condition '70 set by just purchasing it outright at $400 or piecing it together?? Also, as you've stated the first 75% is always the cheapest...Wait till you have to find a Mickey Lolich #715 for under $8
