What is the significance of the comparison of the wooden boards to gold?

The casket Cash is assembling is like gold in value. The wood is yellow like gold.... but its value and craftsmanship is immeasurable. 

The wood is a yellowish color, as “yellow as gold, like soft gold”, but its value is beyond comparison to gold. It is so important to Cash and the Bundren family because that’s where their mother will be laying to rest. It’s value is immeasurable because it means so much to the family. The coffin was most important to Addie, who “could not want a better one, better box to lie in”. The coffin would “give her confidence and comfort”, where she would lay in for the rest of her life.