First contender, Shapeways, representing additive manufacturing on-demand, produced from a polyamide plastic powder (nylon) using laser sintering:
Next up is i.materialize also with laser sintered polyamide:
i.materialize also produces in ABS plastics (like some desktop printers) through FDM:
Ponoko is more expensive on the polyamide:
But cheaper on the ABS:
I haven’t gotten back quotes for US based, small batch injection molding yet. That production does require a minimum run but series can be down to a few hundreds or even tens of units. Injection molded products will be very similar to the ABS material quoted here. And where 3D printed ones run a bit expensive, injection molding gets things to price points as low as:
Calculating the price to home print this also remains. Apart from the issues with product fidelity and finish on home printers I also needs to work out a proper calculation model.
But regardless of the cost, I’m pretty sure the simple fact of laziness means that “very few” (relative to the entire general population that is) will be printing their own gear in the foreseeable future. Most people simply don’t give a shit that they can design and home manufacture whatever they want (as long as “whatever” is a mason jar lid or the likes that is).