Walmart offers the Blaupunkt 300-watt 8" Amplified Car Subwoofer for $68.99 with free shipping. That's the lowest price we could find by $28, although it was $9 less last week. (Most stores are charging closer to $130, however.) It features variable bass boost up to +18dB, variable input gain control, and thermal short and overload protection.
Teaching can be tough, but the life of a K-12 educator depends a lot on where you live.