The exchange rates in Yemen are generally the same everywhere, whether at the airport, the airlines ticket office or at the exchange office in the city. Changing money on the street is possible though you will be charged a small “bakhshish”. Ask about how much it is in advance.
All organized tours to Socotra are more expensive that the ones you could arrange by yourself. You are definitely not going to have time for meals three times a day in the camp. You are likely to have meals only in the morning and in the evening. So, you can order individual breakfast and dinner from camp’s employees. And boiled water is always available for free.