Walmart offers the Pyle 2-Channel 300-Watt Mini Amplifier, model no. PLIPA2, for $18.99. Choose in-store pickup to dodge the $4.97 shipping fee. Outside of the mention below, that's $5 under the lowest shipped price we could find. It features iPod direct audio inputs, 3-way electronic crossover network, and adjustable gain control.
Want it shipped? Nothing But Software has it for $19.57 with free shipping.