Best Buy via eBay offers the Clockwork Synergy 22mm Premium Classic Interchangeable Watch Strap in Black, Beige/Navy, Black/Red, or Black/Gray, model no. (P-PVD)22NL, for $2.99 with free shipping. That's the lowest total price we could find by $12. It's compatible with most Pebble watches, and most watches that takes a 22mm strap.

Of note, it's also available in Gray or Army Green for a buck extra.