After scanning a planet and pressing Y, the planet information screen on
the right reveals if the planet is "Rich", "Moderate", or "Poor". Rich
planets give approximately 10+ good scan for ores. Ore yields vary from
1,500 to 3,000. Moderate planets give approximately 8+ good scans, but
around 35% to 55% yield compared to rich planets. Poor planets give
approximately 5+ scans with yields 5% to 25% compared to rich planets.
Note: If there is an anomaly on the planet (typically a rich planet)
and you land before you scan, usually the Star Map on the Normandy will
put you in a strange location away from the anomaly planet. You must
then go back and rescan to get your ores.