The best price found from United States to Quepos is on April 29th, 2015, travelling with Copa. See more options, compare prices and book: search flights.
Both NatureAir and Sansa service Quepos, which is the base town for accessing Manuel Antonio. Prices vary according to season and availability, though you can expect to pay a little less than US$75 for a flight from San José or Liberia. Flights are packed in the high season, so book (and pay) for your ticket well ahead of time and reconfirm often. The airport is 5km out of town, and taxis make the trip for a few thousand colones (do not pay more than US$8), depending on traffic.