How much are yellow diamonds worth per carat on average?

Anything is 'worth' whatever someone will pay you for it.

'Yellow' diamonds -- the colour, can range from diamond clarity M-Z in colourless diamonds, and from "Faint, Very Light, Light, Fancy Light, Fancy, Fancy Dark, Fancy Intense, Fancy Deep, to Fancy Vivid" in fancy diamond colour.

Every diamond is valued by its cut, colour, clarity and carat weight: of these, there is no 'average'. Generally, however, you will pay less for a colourless diamond rated M-Z in colour than you will pay for a similar diamond rated as 'faint yellow'.