Bulk Formulabs ink runs about $12 for 240cc. Each tank takes about 10cc per fill, so that's 50 cents per tank. Replacing the tank every 6 fills adds about 50 cents to each fill. I get far more than 6 fils per tank, so I probably average 75 cents per fill per color.

The real beauty of Canon is that they are SO easy to fill, it takes about 2 minutes and there's no resetting or chipped carts to worry about.