Useable qty. of ink difference between regular and XL ...

You're right that the quantity of XL ink is not double, but if you check the Canon link that joebassman posted, for the CL-I251 inks, you do get at least twice the printed pages with the XL cartridges.  That is believable since there will be a certain quantity of ink retained by the felt in the both types of cartridge when they are essentially empty, so the usable quantity could be double.

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On your statements about regular size blue (cyan) only being available in a multiple pack, check Amazon - CLI-251C is available in singles.  I think you'll also find pricing for XL is less than double, so (with double the page yield) it would represent some savings.

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I do wonder if there isn't some collusion and price fixing with there being little variation in the pricing from one seller to another (no real bargains in the name-brand inks).

I do notice the "generic" inks are now identified as "compatible" when they're not the name-brand.  Not sure if Amazon requires that now from customer comlaints or if the legal system requires it to be more clerarly stated so they don't get sued by the name brand manufacturers.  It used to be that the generics were pretty much described as if they were the name brand in a very deceitful way.

We get quite a few enquiries about XL ink cartridges as it can be difficult to understand why ink manufacturers produce two such cartridges that are for the same printer but appear to be different sizes. The simplest way to explain this is to say that the XL cartridge (despite the name) is actually the same external size as the standard cartridge but they differ in their internal capacity. An XL cartridge is capable of holding more ink and is, therefore, able to print more before running out. XL ink cartridges cost a little more than their standard sized counterparts but the increased amount of possible prints offsets this cost and makes them a more economical choice in most cases.

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