def price_range
      prices = variants.collect(&:price).collect(&:to_f)
      format =  "%0.2f"
      if prices.min != prices.max
        "#{format % prices.min} - #{format % prices.max}"
      else
        format % prices.min
      end
    end
def price_range
      prices = variants.collect(&:price).collect(&:to_f)
      format =  "%0.2f"
      if prices.min != prices.max
        "#{format % prices.min} - #{format % prices.max}"
      else
        format % prices.min
      end
    end
compute the price range