The thing that generaly makes some heatercores more expensive is rarity. Not quality. This is a part that's designed to fit into various cars. Some models are made by the millions, while another model might only have a couple thousand produced. Just because you buy a used heatercore that might be very expensive if it was new, does not mean that you are getting a better heater core. The
heatercore I mentioned picking up earlier was only $20, from napa, and is just as good as a $100 one, and I don't have to worry about oil on the inside, or leaks. The fins on a heatercore are also
very soft, and can bend easily and lower efficiency. If you get a used heatercore, there is a better chance of these fins being bent.
I say go with a new one. Just my humble opinion, though.