What does milling the head of a motor mean and usual cost of doing so

Milling the cylinder head is removing a very small amount of metal from the cylinder head to engine block mounting surface. This is usually done to true up the mating surfaces and make them perfectly flat so that a head gasket will not fail due to flaws between the 2 surfaces. Just milling the head might cost $100 to $150. The expensive part is the labor for removing the head and the parts cost for gaskets, head bolts etc. Hotrodders have milled or shaved heads for years to increase compression ratio by essentially making the combustion chambers smaller. Your task of milling the cylinder head will remove just enough material to make the head flat.

i would just replace the head it would be cheaper in the long run and for the over heating issue....what temp is your coolant good for and when was the last time the thermostat was checked.