Two most recommended tour agencies by Lonely Planet are Plantours(http://www.plantours.com ) and Senkron (http://www.senkrontours.com). Backpackers.net (http://www.backpackerstravel.net/index.php?pid=26&id=87 ) is recommended by Turkeytravelplanner.
A list of tour and its price is listed below. Tour price is normally tagged in Euro. You can pay by credit card but I found them charging with quite higher currency rate. It’s better to pay them with cash in Euro. Children below 2 are free, and below 7 years old is 30% discount. Istanbul tours fee include transportation (to/from in-city hotel), guide, entrance fees, meals as mentioned and VAT.
Listed below is entrance fee to top major sights in Istanbul. No entrance fee to bazaars, mosques, and tombs. However, at some tomb places donation is voluntarily requested.
Topkapi Palace 20 TL, children free
Topkapi Palace Harem 15 TL
Hagia Sophia 20 TL, children free
Basilica Cistern 10 TL, children free
Archaelogical Museum 10 TL, children free
Miniaturk 10 TL
Dolmabahce Palace, 20 TL, children free
Galata Tower, 10 TL
Camlica Hill, free
Naval Museum, 4 TL
Istanbul Modern, 7 TL
Chora Church, 10 TL
Military Museum, 4 TL
Sufi Dance, 35-40 TL, children under 15 25 TL (including refreshment)