Screen printed silver is the main conductor for cheap printed electronics... Two barely trained operators with a small machine can run a batch of a few hundred thousand pieces within a week with screen/stencil printing equipment. So a bucket of silver paste, some plastic sheets, and a moderately clean space will do. In the meanwhile copper might look cheaper at first glance, but you need to consider the processing cost as well. When considering a single sided circuit with the same volume you'll need at least a few you need hundred square meters of photoresist, large quantities of obnoxious chemicals (degrease, developer, etchant, stripper, plating solutions or OSP), way more space for the lines (degrease, roll laminate photoresist, illuminate, remove protective foil, develop, etch, strip, OSP immersion/spray or plating), not to mention the cost of the water treatment plant assuming you're not running it in China where you can just dump it into the river I suppose. For assembly the circuit boards still have an advantage: solder paste is a lot more forgiving than conductive adhesives. Needless to say, PCBs are well on their way out for certain applications! Obviously copper still has a few major advantages, it's a great heatsink, mechanically more forgiving, able to carry great currents easily, and is very easy to assemble components on. But think about medium volume RFID, keyboards, signs, etc. and you'll usually end up with screen printed silver paste as most economical solution for medium to lower-end high volume manufacturing. For extremely high volumes you could consider aluminium as conductor in more than a few cases, but lets not get into those pesky details!