2019 MARKET UPDATE: CGC 9.8 IS THE BOMB
As one of the few DC comics on the 100 Hot Comics list this year, BA#12 is an interesting study. In common with other DC keys, prices are up and down depending on the grade.
The big difference between DC and Marvel right now is the movie output. DC is slow, Marvel is relentless.
CGC 9.8 is just about the only higher grade to show a profit this year. Harley is hanging on, for now. Recent sales:
2018 MARKET UPDATE: STILL HOT AFTER ALL THESE YEARS, HARLEY
We are still seeing strong sales of this book, though most grades are not returning to the highs of 2016. However, a very recent sale of $1,700 in CGC 9.8 is a definite sign that Harley Quinn fever is still going strong!
Don't let your standards slip. White pages, CGC 9.8, or nothing.
2017 MARKET UPDATE:
She came, she saw, she kicked ass... right until the Suicide Squad movie had been and gone. After which, Batman Adventures 12 dropped in popularity.
Prices have fallen by up to 40 percent from their crazy highs. That said, recent sales have shown an uptick. That's not surprising. She's a character that is not going to go away, and surely there are more movies in her future.
Harley Quinn is the hottest "dark" character in the world of comic books. Way hotter even than Deadpool.
It's kind of a spin-off from the Joker's popularity. Quinn is Joker's girlfriend... kind of (the DC Universe gets a bit squirrelly at times). She's a close ally of Poison Ivy (Batman #181 is also on the Hot 100 list).
Even though this book is not in the DC continuity (again, confusing...), it's considered the #1 must-own book by fanboys, cosplay lovers and comic book investors.
Batman Adventures #12 is a common book. But demand drives value. At one point, CGC 9.8s were hitting $2,000 and more. They have come off the top a little, but are comfortably more expensive than the equivalent grade of New Mutants #98.
Stick to CGC 9.8s. Don't buy a lesser grade, these are way too common. Pity the fools paying $300 or more for CGC 6.0 books.
See also Harley Quinn #1.