I would take Kirk's word for it.
Of the titles I collect (see post above), I would say the ones I'm missing are pretty rare because I've been looking for 5 years on ebay and stores all over the country and haven't found them (I still say they are probably in the closet of someone's little brother because they came in those 20-packs from Toys R Us). The following are also very rare in the titles I collect (I've paid up to $100 in some cases to get copies):
Action 686, 718
Adv of Superman 499
Superman 77, 96
Superman Man of Steel 20
Detective 680, 682 (orange cover), 685, 691
Batman 517, 524
If you find any of these, buy them right away and don't sell them for cheap!
Just to add something as the basis for my statement of rarity. I know three other guys who have tried vigorously to find DCU Universe variants in these titles. The ones listed above and the ones I'm missing have been very hard for the other guys to find as well.