In Drupal, when you create a field, you can specify the default value for a field. For example, if you have a field called price, you can set the default value to be 400. When the user creates a new node, 400 is the default value, but s/he can change it to whatever s/he wants.
Is there a way to link this default value to a node field value.
For example, lets say on node/12 I have a field called "price default". If the user enters 500 on this field (on node/12), the default value of the price field (for all new nodes) will change from 400 to 500.