I think the issue of booking after arriving at the Galapagos has to do with quality. The better (more expensive) trips put on by people like National Geographic, have much nicer ships with nicer facilities. The top end trips also generally have much smaller numbers of people on the ships, better more experienced guides and naturalists who are well-known worldwide. These trips also get preference from the Darwin Institute and in terms of the land experience...smaller groups, more spacings between the groups, etc. Some of the less expensive trips are fine, depending on what you are looking for. Keep this in mind -- it is a once in a lifetime experience --do you really want to "go on the cheap"? There is also well-established risk with the less expensive trips with horror stories about diesel fumes, over crowded and understaffed ships and shore guiding, poor meals, cramped sleeping quarters, and on and on. In a nutshell, you usually get what you pay for, but everyone has a different expectation level.