They didn't (make them in Mexico). They were made in Germany for the Mexicans. These are (IIRC) the "older" small ring design. Fine for low pressure cartridges, but not as strong as later (1898, large ring) ones. But I might be wrong about that. Still 7x57 is a nice cartridge.
$350 is pretty high unless its in really, really good condition, or the "extras" are worth a bunch. Although I haven't seen many for sale lately, maybe they've gone up in value. Seems like the last one I saw for sale at a gun show was $200-250, in pretty good shape.