They are a good "budget" head...especially if they're found cheap. But some people think they are worth their weight in gold and nothing could be farther from the truth...About $300-$350 is a good price IMO.
Some people
DO buy the complete Explorer motors. There's a sensor missing in the lower intake you have to add for a mustang, and the oil-pan, pickup, and front cover are different (Summit or a donor mustang would have the kit). Other than that, it is a good alternative to the standard HO motor. Add a B303 cam and it's off to the races!