c. 1300, "having sound judgment, sane, rational," from Old French raisonable, from Latin rationabilis, from ratio "reckoning, understanding, motive, cause," from ratus, past participle of reri "to reckon, think," from PIE root *re- "to reason, count."
What the majority of people consider to be 'reasonable' is that about which there is agreement, if not among all, at least among a substantial number of people; 'reasonable' for most people, has nothing to do with reason, but with consensus. [Erich Fromm, "The Heart of Man," 1968]
Meaning "moderate in price" is recorded from 1660s. Related: Reasonably.