Math time!
$20 for 6 hours.
Average new PC game at retail? $50
$50 / $20 = 2.5
2.5 * 6 = 15
15 hours for the normal $50 you'd pay? That's better than Doom 3 AND HL2 combined. Hell, it's better than most shooters twice over, with regards to playthrough time.
Anyway, where is this mythical, inexpensive, and massively long *shooter* game you're speaking of, Prozac?