I would like to get 2000 parts of a small 2.5" x 2.5" product made to test market it. It's not economical to have it 3D printed in this volume and it's not economical to make a cheap aluminium mold for a small production run.
Proto Labs has a service called Protomold that is well matched to your application. Aluminum is probably your best bet, and it is fairly affordable. If a few dollars per part doesn't work for you, I have to wonder whether what you are selling is worth making. Plastic things with a value below that quickly end up in the trash, and producing such objects does not make the world a better place.