Which is probably why comics don't really see a big sale spikes when a film comes out.
Depends on the comics in question. The Batman comics got a huge boost from the movie in 1989. Since then superhero comics in the regular DC or Marvel shared universes haven't seen much of a sales boost when movies come out, but comics that are standalones can still get huge sales boosts from films. The Scott Pilgrim books, the Sin City books, Watchmen
, and 300
, for example, sold in massive numbers when their respective movies were released. So even movies that underperform at the box office, such as Scott Pilgrim vs. the World
, can drive book sales way above their usual level.