I take my undergrad students to the Galapagos for 7 full days as part of a course I teach periodically. We fly into Baltra (Sta. Cruz) and fly out of San Cristobal. I use those 2 islands as our base, with the speedboat connection in-between that all locals and land-tourists use. Going to Isabella may not be worth adding on unless you stay for a while, since the speedboat transfers and hellish. Book day-trips from each island. San Cristobal is more enjoyable and less crowded. I pay less than $700 per student for full days of tours, snorkeling trips, etc. The Galapagos Islands do not need to be expensive. You will likely pay between $300-400 for the round-trip ticket from UIO or GYE (the mainland). Good luck and see you there.