reasonableness has 6 distinct synonyms spread across 3 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the state of having good sense and sound judgment
“his rationality may have been impaired”
goodness of reason and judgment
“the judiciary is built on the reasonableness of judges”
the property of being moderate in price or expenditures
“the store is famous for the reasonableness of its prices”
moderation in expectations
“without greater reasonableness by both parties we will never settle this matter!”
the quality of being plausible or acceptable to a reasonable person
“he questioned the tenability of my claims”
Synonyms, antonyms and the broader and narrower relations for reasonableness come from Princeton University’s WordNet. Full sources and licences.