All4Cellular via Buy.com offers the Kensington SoundWave Sound Amplifying Car Mount for iPhone, model no. K39255US, for $19.98. With free shipping, that's the lowest total price we could find by $8, although we saw it for $10 less after a rebate in Febraury. This mount features an amp that doubles the volume of your iPhone's internal speaker. Although pictured, the charging cable isn't included.