Buyukada island is one of the 9 islands in Marmara sea. These islands are usually known as Princess islands to westerners and are home to filming many Turkish soap operas. The Adalar ferry ride from Kabatas station in Istanbul takes about 1 hour and 45 minutes to get to the island and it is enjoyable. You can enjoy the beautiful scenery and drink some Turkish tea. The island itself was full of gift shops and restaurants, which categorizes this island for me as a tourist trap. The ferry schedule forces you to eat lunch there since the earliest ferry to leave the island is at 3 pm. Despite me being not picky at all about food and also using Trip Adviser and internet for finding restaurants, the food was expensive and its quality was not good (another sign of a tourist trap). Carriages with horses can take you on a tour of the island to see rich people's vacation homes and soap opera filming locations for 70 TL (about $35).
One expense that you can avoid is the cost of tours to visit the island which can vary but it was offered to us for around $65 per person. You can ask your hotel staff how to get to the tramway. You can then take the tramway towards Kabatas and get off at Kabatas station. Then take the ferry very cheap. If you buy an Istanbul Card beforehand (as soon as you get to Istanbul) then you can use the public transportation. You can charge your Istanbul Card in tramway stations, metro stations, and where ever you see the word "Akbil". This works as your bus pass. It is enough to have one bus pass for your entire group of people. The pass itself is 7 TL ($3.5) and each bus ride is about 1.95 TL ($1) per person. Remember that there are many ferries but there is one that works with your bus pass (it is part of transit system) and is very close to Kabatas station. You should look for "Adalar" ferry. The ferry usually leaves at 10:30 in the morning and comes back at various times. You can check the schedule either online or at the ferry station. The ferry stops several times at different islands. The Buyukada island is the last stop of the ferry.