I managed to get Inksaver from Serif Extra for �9.99 with a working code. Unfortunately, they are sold out and are not going to get any more (may be a reason why - see below). The Amazon one seems to be a rip-off - read the reviews carefully.
I used Inksaver for quite a while and it was very good BUT ... I hadn't printed across my wireless network for a while. When I came to do this, I think that Inksaver had messed with my settings and stopped me printing. I've now uninstalled it (pity!).
I might revisit it sometime, I'm sure I was saving quite a lot of ink.