For Prime members only, Amazon offers the Braun Series 5 51S Foil & Cutter Replacement Head in Silver for $19.80. Order via Subscribe & Save to cut the price to $18.81. With free shipping, that's $5 under our mention from last October and about $3 under the price you'd expect to pay at your local store. It fits Braun Series 5 590 and 550 models.
Be sure to cancel Subscribe & Save after your order ships to avoid being billed for subsequent shipments.