Angel, it depends on your needs I suppose. But it does not need to be large pieces to make it economical. I have cast up AFV Club British radio components for other kits that do not have a radio. The cost is 100 times less the cost of buying a resin or accessory kit. The same with British Fire extinguishers. Instead of the blobs that come with kits, I have a mould of a great casting that I use. Flimsies, fuel cans, lamps, etc.
Same for German jack blocks, extinguishers, Notek lights, etc
I have cast up ammunition for the K9 Rail gun for a munitions panzer - no seams on a turned one.
I have also cast up small parts to replace malformed kit ones or get better detail than in a certain kit - I.e. Tamiya Cromwell wheel Hubs and their Churchill cupola for the Bronco Comet.
The cost of the mould making agent is not that expensive if you keep the moulds small and have minimal wastage (just make sure the side walls are strong enough to hold shape). I use Lego block frames pressed into artists clean clay to form the mould. Easy and you can make any size you want.
And yes make up moulds of any boxes, crates, etc you have, and when making other things any excess resin can be poured into the box moulds for future use.
and remember, you make a good master and cast it then Kalin may buy a few off you and share the cost!!