Baby lost 10% weight... Should I supplement?

So we're going home from hospital today. My son has lost 10% of his Body weight. He's been great at latching and breastfeeding so far, so I have him 4 oz of donors milk which he devoured. They want us to feed him formula at home until my milk comes in, donor breast milk is $50/4oz (were beginning his second full day of life) but the formula and bottle feeding scare me in the idea that he won't want to breastfeed anymore. But he's been great about it so far... And most importantly I want my son to be healthy. What has you experience been? Any abide?