Finding synonyms for the phrase "good sum" requires understanding the context in which it is being used. For example, if referring to a financial transaction, "substantial amount" or "generous sum" could be appropriate synonyms. If discussing a pleasant surprise or outcome, "welcome surprise" or "positive result" might work. If referring to an impressive amount of knowledge or skill, "impressive proficiency" or "expertise" could be used. Other synonyms for "good" that could be paired with "sum" include "satisfactory," "excellent," "competent," or "worthy." Overall, choosing the right synonym depends on the specific situation and the intended tone and meaning.