My sister-in-law was generous enough to pump extra milk for Pia, so we started supplementing some with breastmilk. Pia gained weight, but she was still below 6 lb. Eventually I reconciled myself to the fact that formula would be necessary. Pia continued to receive some breastmilk from Amber and me, but she needed more than that.
Once she was finally getting enough to eat, Pia started to thrive. It took her three weeks to regain her birth weight (6 lb, 5 oz), but she reached 8 lb about 10 days after that. She slept a lot better once she wasn't hungry. Her skin lost its red hue.
Based on my experience, then, formula seemed to be some kind of miracle drug. Breastmilk alone left my baby red and sickly, while with formula she put on weight and gained strength. All for a mere 8 cents per ounce. I really don't understand how this is considered expensive, especially compared to purchased breastmilk, which retails for $2-$3/oz. We pay less than $2 a day for formula, and we're happy to pay it.