I am just getting to know some OSPF and its trickery, but I wonder if the NBMA-neighbour is basically the same as the usual OSPF interface then how does one set the NBMA-neighbour cost? Or is it indirectly calculated from some other parameter?
Its just to set cost, at the interface. It does not matter if its nbma ore something oher. Only difference is to set priority at nbma neighbor (and most likely put the same cost at other sides interface)
NBMA... its mean you can built your OSPF Network manully like you can specify which router will the Designated and which router will be Back designated..
There is more detail about you can find in the WiKi too...