We booked round trip flights on Avianca for $371 per person. That was for "L" class tickets, which allowed us to change the flight if necessary, as well as choose seats in advance and have a bit more luggage allowance.
Heading to Baltra, the flight leaves Quito at 7:51am and arrives in Baltra at 10:48am. I believe it makes a stop in Guayaquil on the way. It's going to mean a VERY early wake up that morning since we have to transfer to the airport, then get the tourist cards, do the luggage inspection thing, and then actually check in and get through security.
Returning to Quito, it leaves Baltra at 11:33am and arrives in Quito at 4:15pm, and again, I believe it makes a stop in Guayaquil on the way.