Maybe the price difference isn't that great anymore (though it was definitely noticeable when I last shopped a couple years ago), but the size difference is. Black and white lasers have one toner cartridge and drum, but color lasers have four and usually a large transfer belt, in addition to other complexities that usually make them much larger. That being said, the Phaser 6010 is surprisingly compact.
However, if you're looking only for monochrome printing, I once had a Brother HL-2140, which they don't appear to make anymore but seems to have been replaced with similar small models. Mine didn't have duplexing (the main reason I replaced it), but something about the same size and a similar price now seems to be available as the HL-2240D. I haven't used that exact printer, so I can't say anything for sure, but it looks like a nice, compact monochrome laser if that's what you're looking for.