TIL that game has a rumored budget of 2 billion.

Sometimes, when I play a AAA game and something expensive is visible on screen (e.g. half of New York getting destroyed during that long quick-time event in Spider-Man), I like to shout "Production value!" at nobody, like that director self-insert kid in "Super 8" (2011).

I get a feeling I would ruin my voice doing this every time in GTA 6.

To answer your question, I think we would have to look at what music licenses usually cost. Some quick googling tells me that $7500 is hardly an outrageously low sum for a song from a middle of the road '80s band. They aren't exactly Depeche Mode. I think they would have benefited far more from the inclusion of their song in this game financially (since it would cast them into the limelight again, providing streaming revenue and perhaps gain them new fans) than the little and likely very temporary publicity they gained from rejecting the offer.