For a broad reference and example, according to bluenile.com, for a very nice 3 ct round diamond (very good cut dimensions, at least F in color, and at least VS1 in clarity) the price tag can range anywhere from $80k to $200k.
http://www.bluenile.com/diamond_sear...ss&filter_id=0
Of course, if the bar isn't this high for cut, color, and clarity, the cost can be much lower.
Yep, that's a nice Cobra alright!