My friends and I run T3 and T4 Captures when we need ducats. There is a fairly good chance of getting the Akbronco Prime Link, which is worth 50 ducats at the trader. If you don't have any T3 Captures, you can get them fairly often from doing T1 Defense. If you don't have those, then my favorite place to farm T3 Captures is on Cambria, Earth. This also happens to be a great place to farm for other T3 keys, but your best bet is to do nothing but void missions so you're always getting parts.
PS: Prisma Gorgon, like the others, is not worth using other than for mastery. It may even be cheaper for you to pick it up on the trading channel.