When mine went 8yrs. ago in the dead of winter, they were priced at $150 used -$350 new (not sure about now). The heat control valve wasn't cheap either. Instead, I took the heater out of the coolant circuit and got better gloves, but really suffered that winter (there were blankets for the passengers).

In spring I hit the junkyards and most of the older ones I could find were similarly corroded. I dug a couple out of newer cars (1800ES and Saab 99 - about 9 yrs. younger than a '66) and had pressure tested at a local radiator shop. The later heater cores have a drain plug on them and the later heater housing has a hole to accomodate this -- the '66 housing does not have a drain plug opening so the late heater core plug hole must be sealed to use it in an early housing (also done by radiator shop).

All told, about $45 to test and convert a good used later one for early heater box use. . . . $35 less if you are willing to drill a hole in your housing.