What else give almost 100% discount/Rebate?

I have enough workshop with adjacency bonus.
It seems that I can build effectively infinite whatever I want for $1 each. Firestorm, any vehicles. However, guns and instantly build items cost remain the same.
Am I correct?
If so, what turns it on?
The workshop or the engineer?
Here are some references that's not clear http://xcom.wikia.com/wiki/Workshop
It says that vehicles, facilities, and foundry updates are discounted. However, weapons like EMP cannot is also discounted. It seems that anything that took a while to produce is discounted.
Also it seems that the CONNECTION between workshops and not the workshop itself that turns this on.
I just want to make sure.
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