For many years, certainly during the seventies, the National League used to count people-in-the-seats while the American League counted tickets sold. I’m not sure whether this was still the case for the NL in the late 80s, but it seems very probable that in the games you’re talking about 13k represented 13k tickets sold–so, likely fewer than that in the stands.
Last September, at the end of a lost season for the New York Mets, I paid the princely sum of $1.79 for a ticket to a Marlins-Mets doubleheader at Citi Field (secondary market of course); fees pushed the price to a whopping seven and a half bucks. Naturally, transportation added significantly to the total cost…
Anyway, the official attendance for the DH was 24,966, which may have been the total number of tickets sold but was off by two orders of magnitude if we’re looking at fannies in the seats. I doubt there were 500 fans in the stands when the first game began, and if there were more than 3,000 by the time it “filled up” I’d be shocked.
The atmosphere was…about what you’d expect. It didn’t help that Mets fans were outnumbered by, or at least outcheered by, Marlins fans–probably the only time I’ve experienced this at a major league game. Based on that, I have no trouble believing that “fans of the other team” may equal or even outnumber A’s fans at Oakland’s home games these days.
I attend a bunch of minor league games in a 5,000-seat stadium near where I live, and 3,000 fans in a 5,000-seat stadium can make for an excellent atmosphere–loud, bouncy, upbeat, and enthusiastic–but 3,000 fans in a 40,000-seat stadium just can’t.