Man, what are you guys talking about? There is NO comparison between trying to acquire the first 15 issues of Flash in his own title (105-120) in HG and trying to acquire the first 15 issues of GL in his own title (1-15) in HG. Flash is exponentially harder. Flash only starts getting relatively "easy" in the 120s, and those issues are still a b-itch, even after the recent surge in supply.
I was basing it on my personal experience when I was trying to put Flash and GL runs together a couple of years (or three) ago. I guess the difference here is that when I was looking there were plenty of HG Flash issues available even into the early teens. I landed 5 HG Flash issues (all grading between 8.0 - 9.0/9.2, I believe) and 1 HG GL (#7 in 9.2 that I sold later ) in one quick purchase, with two of the Flashes being tied for top census (#128 and #135 or #139, if I recall correctly) at the time.
I will definitely defer to your wisdom here though, Tim, since you know the HG DC market much better than I definitely do.