Affordable 3D printing of reflector
I would not recommend extrusion printers for this, because they are unlikely to produce a smooth enough surface.  To get a clean surface, the irregularities have to be a fraction of visible wavelengths, which is to say on the order of 0.01 micron.
Without knowing what sort of reflector you're thinking of (flat? spherical? parabolic?), it's hard to recommend a specific optimal, cheap approach.  All in all, you're probably best off looking in standard catalogs such as Edmund Optics.