Is it really worth making a mould for making 500-1000 units?

At those kinds of quantities, I would just fire up the 3D printer, and leave it printing in the corner of the office for a few weeks (with an auto-ejector).

It would end up costing ~10 cents/part, assuming you already have a 3d printer for prototyping and already have someone who knows how to load a new reel of filament into it every morning and evening.

Thats hard to beat for anything under 10k units...