As of Firmware 6.3, you can configure the OSPF cost of each interface and PepVPN Connection using advanced mode. To do so, please navigate to Network > OSPF and select an area or create a new one.
From there, click the question mark icon next to Interfaces
and than click the link that appears.
Then, select the interfaces you want to establish custom costs for and then specify the desired OSPF cost.
Click Apply Changes
for your changes to take effect.