SoltechSecurity via Amazon offers the Samsung Secured Digital Wireless Dual Audio Baby Monitor, model no. SEW-2001W, for $35 with free shipping. That's the lowest total price we could find by $15, outside the offers below. (For further comparison, in September we saw a similar set that included an extra parent unit for $10 less.) It features a 900-foot range and voice activation.
A couple of close price: Fry's offers it for $37.99 with free shipping, and Kohl's offers it for $29.99 plus $8.95 for shipping.