Departing from San José, NatureAir and Sansa have daily flights to the Drake airstrip, which is 2km north of Agujitas. Prices vary according to season and availability, though you can expect to pay around US$105 one-way.
Alfa Romeo Aero Taxi offers charter flights connecting Drake to Puerto Jiménez (US$430), Carate (US$450) and Sirena (US$420). Flights are best booked at the airport in person; if there are several of you, one-way fares are typically less than US$100.
Most lodges provide transportation to/from the airport or Sierpe, which involves a jeep or a boat or both, but advance reservation is necessary.