Where to stay and cost traps: check the fairs, the station district honestly
Frankfurt has more than 50,000 hotel beds for business travellers, and that sets the prices: during the week without a fair you pay CHF 110 to 150 (about EUR 118 to 161) for a mid-range double room, at weekends often less because the bankers are gone; Frankfurt is one of the few cities where Saturday is cheaper than Tuesday. In fair weeks prices triple: the Book Fair in October, the IAA every two years in September, Ambiente in February, Light and Building, Achema. Check the Messe Frankfurt calendar before you book, or you will pay CHF 400 for an Ibis. Good locations are Sachsenhausen, the Nordend, Bornheim and the Westend, all quiet and ten minutes from the centre by U-Bahn. The Bahnhofsviertel, between the Hauptbahnhof and the city centre, is cheap and practical but, honestly, the toughest district in Germany: an open drug scene on Taunusstrasse and Niddastrasse, red light, homelessness, and all of it visible, even by day. It is rarely dangerous but often unpleasant. Kaiserstrasse itself is fine, with good restaurants and bars, and the district has the best Asian food in the city, but with children, or if that sort of thing bothers you, stay somewhere else. One cost trap is eating right on the Römerberg, the second is the airport: if you only want to get into town, take the S-Bahn for CHF 6 instead of a taxi for CHF 40.