I have cisco 3750 with three routed interfaces. IntA have an auto-assing cost of 1, IntB have an auto-assing cost of 10 and IntC have a manually-assing cost of 100. If IntB goes protocol down, I will like the device to automaticlly increment/decrement the cost of IntA and IntC, in such a way that IntC get the routing path preference.
The cost on the interface may only be a small part of the total cost for using the interface. The cost to a destination is cumulative, and the local costs may only be a small part of the total cost. You simply cannot tell which interface is preferred for a given destination just by the local cost on an interface. For example, the interface with the cost of 100 may be preferred if the total cost to the destination is smaller than the total cost to the destination using the interface with a cost of 1.
In any case, you would need to use something like an EEM script to change the cost of an interface when a different interface goes down, but you really cannot tell from that which interface will be preferred for a given destination without looking at the total cost for each path to the destination. You can check that in the OSPF database.