The issue of the various options for fountain pen ink stimulated my curiosity, so I did a little research. The following discussion summarizes my findings.
Let me start by saying that I only looked at name-brand products available from reputable on-line retailers. I know that it is possible to purchase very inexpensive ink, especially in cartridges, from vendors such as e-Bay, but I am hesitant to assume that such anonymous products are equivalent in quality to the major brands. Ultimately, ink isn't really all that expensive, so I don't think it makes sense to take unnecessary risks by shopping the obscure brands. But obviously, that's my opinion - your mileage may vary.
Also, because I didn't want to take the time to do an exhaustive survey, I know that I missed some very obvious major brands. And I only looked at selling price from one retailer - it's possible that there could be minor variations if multiple retailers were considered. And that's why I have chosen to not identify the specific manufacturers represented in the results. My objective is to arrive at generic answers rather than definitive conclusions that might be true today, but subject to change due to market volatility.
My major conclusions are summarized in two charts:
Ink is comes in either cartridges or bottles. Cartridges are either proprietary, which means that they (generally) only fit pens made by the manufacturer who produced them, or so-called 'international standard' cartridges that fit pens from a number of manufacturers. Most fountain pen kits are intended for use with international standard cartridges. While 'international standard' cartridges generally all have the same capacity (0.77ml for 'short' cartridges, and 1.4ml for 'long' cartridges), proprietary cartridges contain however much the manufacturer chooses to design them to hold. So the only real comparison is unit cost (cost/ml).
So - most ink in standard international
short cartridges cost between 60 and 80 cents per ml. There are a couple of brands that are more expensive. I suspect that the main reason for that is that the manufacturers of those inks are attempting to establish a market placement as a premium product. Those inks are made by pen manufacturers who also are known for more expensive pens. Ink in standard l
ong cartridges ranges from $1 to $1.20 per ml. There are relatively few pens that can accept the long standard cartridges, so my suspicion is that the higher price is due to lower sales volume, and perhaps also less vigorous competition since there are so few manufacturers who offer ink in that size. Incidentally, pens that can accommodate a long standard cartridge typically can also hold two short cartridges a the same time, one in use and the second in reserve.
Pricing for proprietary cartridges is all over the map, but its important to recognize that because these are proprietary, manufacturers have essentially no competition and can charge whatever the market will bear. This is not at all unlike Gillette razors - if you buy their razor, you are locked into buying their blades.
If you look at the standard international short cartridge market, where there is true competition, the least expensive options among those I looked at are Diamine, Kaweco and Monteverde (all of which are very good inks).
Bottled ink also comes in a wide range of package sizes, so again unit cost is the only viable comparison. In my survey (which is admittedly not exhaustive) the average unit cost of bottled ink was 34 cents per ml - which is less than the unit cost of any of the options for ink in cartridges. This illustrates the point that has been made in this forum many times - ink is more expensive in cartridges than it is in bottles, which is why many users opt to use refillable converters (pumps) rather than cartridges. The least expensive bottle inks are those from Noodlers, Diamine and Monteverde. One other observation that isn't obvious from the charts - the least expensive bottled inks are generally packaged in the most ordinary bottles. Or saying it differently, when you let the shape of the bottle dictate which ink you buy, you will likely pay more for the contents (which is often true of products at the supermarket).
Obviously, users have a choice between cartridges and bottles. Bottled ink is less expensive, and there is a much larger selection of colors and ink characteristics. Cartridges offer convenience and are generally less messy, and that's not an insignificant consideration for many people.
Finally, while I didn't consciously try to analyze ink prices versus characteristics, my sense is that the major factors in determining the price we pay is the way it is packaged (bottle versus cartridge) and the name of the manufacturer. If there was any pricing differentiation based on things such as shimmer, shading, water resistance and permanence, reaction to UV light, etc, I did not see it.