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.

The best I can do is a <five pack> for $7.34 or $1.47 each. If I could've linked you a single refill for that $2 mark I would've, but naturally it's not always super feasible to get a single $2 item without paying $5 for shipping. So, hopefully that $7 risk for the bulk buy is helpful for you. Otherwise, I'd just recommend picking some up if you ever find yourself buying notebooks, paper, or other stationery items.