This is a comparison of 0.3ct ideal cut, F color, VS1 clarity princess cut diamonds (click the dollar amount to be taken directly to the diamond):

James Allen: $420

Abazias: $455

Blue Nile: $587

These diamonds are independently inspected and graded by GIA/AGS, which means you can trust their quality. The quality of these diamonds is identical--the only thing that is different is the price.