How much should a round cut 1.5 karat diamond cost? 5k? also what size is the normal engagement ring?

You should really consider the woman's personal taste more so then what ever the average is when selecting a ring. Many women do not like huge rings where others my love them. Keeping her tastes in mind the price of a 1.5 carat diamond would very depending on the CUT, Color and Clarity. For a well cut diamond in that size range you might spend up to 19K. For a poorly cut 1.5K diamond you may spend as low as 7,000. Price ranges depend on which lab has certified the diamond. EGL will be the most lenient of labs. AGS will be the most demanding of labs. So if you find a G color EGL stone, you know that it would be graded by AGS as an I.

So, a 1.5 is going to cost you between $1500 and upwards of $7000 depending on the quality and cut of the stone. There is no normal size for the ring as it really does depend on the woman's particular tastes which should ALWAYS be considered. She's the one that will be wearing it day in and day out.