Amazon offers the Astra Double Edge Safety Razor Blades 100-Pack for $8.30. Add it to your cart via Subscribe & Save and it falls to $7.88 and bag free shipping. That's $0.08 per blade and the lowest total price we've seen for these razors. (It's a current low by $5.) These blades fit all double edge razors.
Be sure to cancel Subscribe & Save once your order ships to avoid being billed for subsequent shipments.