how come the cheapest ticket online (www.thalys.com) from Cologne to Paris is Euro 59 and the RailEurope price is $101? That's quite a mark up.
While it's true that RailEurope's fares are inflated compared to the fares you can get by booking directly with the various national rail companies for any route in Europe, in this case there is another reason for the difference. Booked at thalys.com, the standard 2nd class fare for Koeln-Paris is 88.50 EUR. The 59 EUR fare is a special discounted fare called Smilys. If you were to do a dummy booking, you would be given a single (one-way) fare of 29.50 EUR, but Smilys tickets are only sold as returns (roundtrip). People going one-way simply book an arbitrary return date and throw away the unused portion. You can book up to 90 days in advance. Smilys seats are limited and sell out fast so book as soon as the 90-day window opens.