The "Heintextil" connference is taking place at the Messe Jan. 13-18, so that is probably what is forcing up room rates.
Other than staying out by the airport, another alternative is to stay in nearby Mainz - it's an interesting town on its own (it sits directly on the river) and it is well-linked to Frankfurt proper by the excellent RMV regional transit system (RMV). The Hyatt and Hilton in Mainz both show good availability for your dates.