Crutchfield offers a pair of Polk Audio 5.25" 90-watt 2-Way Car Speakers in Black, model no. DXi525, for $44.99 with free shipping. That's the lowest total price we could find by $15. Features include 3/4" pivoting liquid-cooled dome tweeters, built-in 2-way crossover network, and 60 to 22kHz frequency response.
The Microsoft founder -- and richest man in the world -- also suggests one great book to read.