First, I am a cheapskate so I do not want to pay $7 for a box of six ink cartridges when I can get six of another brand for a dollar or two. Second, even those of you who use gourmet inks for writing letters may be happy to save some money (to buy more gourmet ink!-) on the ink you use for grocery lists.
You might ask why cartridges rather than the bottled ink much more popular at FPN. That's a long story, much of which may not apply to you, but a general reason is that there are a lot more bargains available in cartridges, perhaps because they are so overpriced here in the US of A. It's not too hard to find ink cartridges from overseas on eBay for about the sales tax you would pay on even non-gourmet cartridges in a pen store. There are some catches, of course. The quality could be poor, but that's been unusual in my experience. Shipment could take a while. The range of colors could be limited, ranging from a selection as sparse as that available in Cross cartridges, to a palette similar to other major pen manufacturers such as Sheaffer, Parker, Lamy, Pelikan, or Waterman.
So let's begin with the ongoing saga of the Thornton inks that have been on sale at ridiculously low prices this summer.
These inks can be variable in quality, so you'll need to check that thread for details, but I can give a good recommendation to the blue which you can get in Lamy-shaped cartridges (that fit many Parkers, Heros, and Wing Sungs as well) for $10.22 per hundred, or about one-eighth the going price for other brands. The link also includes color samples of bottled Thornton ink contributed by Amberlea Davis.
I hope to add reviews of other cheap inks to this thread over time, and welcome others to do the same, or share their experiences or opinions of various cheap inks they have tried.