quote:

Paris to Frankfurt by overnight train starts at just 40 euro (£29) one-way in a seat (night travel in ordinary seats not recommended), 50 euro (£36) one-way in a 6-berth couchette, 70 euro (£50) in a 4-berth couchette. Returns are twice the one-way fare. These fares are available if you book online at least 14 days in advance via www.voyages-sncf.com.
Paris to Frankfurt by daytime train costs 81 euros (£58) one-way full-fare, 162 euros (£116) return.

How to buy tickets (via Paris):
The easiest and cheapest way to book is online via www.voyages-sncf.com. The English button is a UK flag at lower left. You must book in two stages, because if you try to book London-Frankfurt in one go, the cheap Paris-Frankfurt fares will not show up: First book Paris to Frankfurt (the station is Frankfurt am Main Hbf (DE)) and back, using the train times above as a guide - it may help to tick 'direct services only'. For 4-berth couchettes or a single-berth sleeper you need to select '1st class'. For 6-berth couchettes or berths in a 2-berth sleeper you need to select '2nd class'. When you have booked this leg, click 'add another ticket' and book the Eurostar London-Paris and back. Make sure you allow at least 1 hour between trains in Paris to allow for any delay, to walk between stations (15 mins) and for the 30 minute Eurostar check-in in the return direction. Remember that if travelling overnight, your date of travel from Paris to London will be the day AFTER your departure date from Frankfurt.

If you prefer to book by phone, or if you have problems using voyages-sncf.com, call Rail Europe on 0870 5 848 848 (lines open 08:00-21:00 Mondays-Fridays, 09:00-18:00 Saturdays, 10:00-17:00 Sundays).