How much is an 8 carat yellow diamond worth?

Today, the only yellow diamonds available for sale on the internet in that carat-weight range with GIA certificates are are:

• 5.03 Carat Fancy Yellow, Cushion, VVS1, US$56,928
• 11.01 Carat Fancy Yellow, Radiant, VS1 US$252,900

So, the diamond you want to buy will cost you somewhere in that range.

If you're selling such a diamond, best practices dictate that you pay for a GIA certificate, which will give you an approximate value of the stone, based on its individual characteristics, as above.