Brace yourself...
In the US, we used Tidy Cats Crystals - we used a small (3.5lb?) jug each time and changed it every week or so (we have two very smelly cats). So we were looking at $6-7 every week, or around $24-28 a month.
The only crystal litter I've been able to find here is one called "City Cat" which is available at Pets at Home - it only lasts about three days and costs £4 per 3.8Lbag! It just doesn't seem to do the same job in terms of dehydration and odor control, though it still works loads better than clay litter....
Our litter expenditures have shot up from around $24-28 (£12-14) per month to nearly $80 (£40) per month!
We're actually considering asking Skyco or one of the other import companies to import a pallet of Tidy Cat Crystals for us - it'd cost a pretty penny, but it's not as if it's something we're going to suddenly stop using, you know?