To read a value from a dictionary, use either dictionary[key] or dictionary.get(key). This will return the value/object that was put in the dictionary on the given key.
Assuming "plant_stats" is a dictionary, "plant" is a key, and "SunCost" is a field on a plant object stored in the dictionary, try the following:
$Label.text = g.plant_stats[plant].SunCost
If that doesn't work, please provide some more information on what you store in the dictionary.