I mean, absolutely cheapest? No regards to quality. Just cheap as hell.

I'm sure I could find something cheaper, but this is $7 for 30--about 23 cents per. Is it good? No idea, but it's got 4.5 stars.

If you want something with brand name, though, Parker refills work out to $1.55 per refill if you buy in bulk. Ohto Flash Dry is slightly cheaper in packs of 5.