Or you could hotwire a room: TripAdvisor's #1 business hotel and #5 hotel overall in Frankfurt, the Moevenpick, is frequently available for $49 ($65 including taxes) and is easily identifiable (although Hotwire is an opaque booking site, using the lists of previously-identified hotels availabe on sites such as betterbidding.com means you can nearly always identify the candidates before making your decision). It's a short walk or a less than €10 taxi from the Hauptbahnof, and the S Bahn between city and airport is fast, frequent and cheap.