Sunsmile*2013 via eBay offers the 33" Studio Flash Lighting Umbrella in Silver/Black for $4.92 with free shipping. That's the lowest total price we could find by $3. It purports to provide about 50% light transmission over a wide, even field. Note that this deal is for the umbrella only; the pictured stand and light aren't included.
Note: This item ships from mainland China and may take three to eight weeks to arrive.