100$ or less subs

[basite]

a real sub, under $100

you must be kidding, right?
cheapest you can go is (new) DIY, or used, but really, even then, 100? do you expect that a sub from that 'price class' can even produce sounds lower as the noise your speakers make?

seriously,
save on, then come back.

Greetings,
Bert.