To add to Pete's comments:
A 'standard short international cartridge' holds about 0.87ml of ink. A 'standard international converter' hold about the same volume of ink and therefore will yield about the same amount of writing. However, ink in bottles is far less expensive than ink in cartridges. For example, Goulet Pen Company (a large on-line supplier of pens and inks, and great people to do business with), sell a box of Diamine Saphire Blue ink cartridges for $8.50. That's 47.2 cents per cartridge, or 54.9 cents per ml. They sell the same ink in a 30ml bottle for $7.50, or 25 cents per ml. So the operating cost is much lower with bottled ink and a converter compared with cartridges., and there is a possibility of a far broader selection of ink colors and characteristics to choose from in bottles. On the other hand, cartridges are easier to pack for travel than a bottle of ink, and can be less messy.
Diamine is a very good ink made in England and about the middle of the pack in pricing - there are less expensive inks (Noodler's is a brand of American made inks that offers an extremely broad selection of colors and characteristics, but only in bottles, and most varieties sell for about 15 cents per ml). Pilot Namiki, a Japanese company, has inks that sell for as much as 40 cents per ml in bottles. Interestingly, the difference in pricing is often more related to the elegance of the packaging; Pilot bottles are beautiful and custom made, while Noodlers uses standard commodity-grade laboratory sample bottles for their product.
There are also proprietary cartridges that have different volumes, and also the relatively rare 1.66ml 'standard long international cartridge' that fits a few commercially-made pens. As far as I know, all kit pens use either 'standard short international cartridges' or standard international converters. Some commercial pens can hold either a converter, a long cartridge, or two short cartridges, one in use and one spare.
Pete mentioned Private Reserve. Unfortunately, Private Reserve is struggling. They were known for some excellent, highly-saturated inks at reasonable prices. However, the owner of the company died, and there was a period of several years when the business was in limbo. There is now a new owner who is trying to get the business re-established, but the reports I've read suggest that there are still problems.
This suggests one other factor - some ink manufacturers are also pen manufacturers, and are fairly large. However, some, such as Private Reserve and Noodlers', are classic small businesses. Noodler's, for example, is one guy who makes small batches of ink in the basement of his home in South Dartmouth, MA. The inks are innovative - highly saturated colors, and interesting characteristics (including highly permanent inks, inks that won't freeze in the winter, and inks that glow in response to UV light), but there can be inconsistency between batches, and supply backlogs are fairly common.
Probably more than you wanted to know - - - but still just scratching the surface of the fascinating subject of fountain pens and inks