OEM parts are just expensive in America. The way it works is Epson manufactures most of the parts for Mimaki, Roland and Mutoh which is the first time they get sold. Then Mimaki, Roland and Mutoh Japan sells the parts again to their American counterparts. Then the American versions of Mimaki, Roland and Mutoh sell the parts again to the dealer who then sells them again to you. That's 3 middlemen by the time it gets to you. By the time the dealer gets their hands on them, the margin is so low that they can't discount the price very much. That's why we sell a mix of OEM and non-OEM parts. Some parts are just better OEM but a lot of the time you can save a lot of money using proven after market parts.
Also, contrary to popular belief, there is no such thing as an after market print head. They all get manufactured at Epson and go out into the world from there. There are no other people manufacturing them and there is absolutely no difference between a Roland, Mimaki and Mutoh head. They are all the same other than how they mount to the machine.