Let's suppose you had started collecting comics in the late Silver Age, and managed to get all the Detective Comics from 327-up. And you had also managed to snag all the Batman Annuals that DC printed up until the end of 1970 (and the reprint of the Detective #27 story in Detective #387). What would your collection of Batman stories in his first 300 appearances in Detective look like?
As you can see, you'd be making pretty good progress on the Batman Detective stories from 200-299 (37 issues) but outside of that you'd have a long way to go. I'd have to look at how much this changed in the next few years after 1970; I remember for example that the Two-Face stories in Detectives #66, 68 and 80 were reprinted as were the stories featuring the Monk from Tec #31 and 32. But I am pretty sure that the overwhelming majority of Batman Detective stories under 150 were never reprinted until the Archive Editions came along.
BTW, as a teenager I actually did track this stuff; I had a little box of index cards that I used to keep track of what stories had been reprinted from which issue. I remember that there were a couple of issues of Batman from the 1950s where all the stories in the issue ended up being reprinted (for example, Batman #125).
Update: Commenter Chris reminded me of DarkMark's tripod page where a whole slew of DC reprints have been catalogued.