Train fare Amsterdam to Cologne

The fare will probably be around 65 euros. But it's hard to say for sure as high-speed trains throughout Europe use airline style pricing' the sooner you book the cheaper it will be.
If you play around with this site (http://www.nshispeed.nl/en) you can get fares and schedules.

Fares vary from € 19 to € 65. Sometimes you can find bargains to Oberhausen, Duisburg, Düsseldorf and Köln with little advance.

It will vary depending on if you want to string together a group of regional and intercity trains (cheaper) or take the direct highspeed ICE that departs only a few times a day (most expensive, when bought on the day of travel).

Tom, there are no regional trains travelling between Emmerich (D) and Arnhem (NL). Only the ICEs.

Any other option for travelling with local trains would involve several changes and a annoyingly slow route via Venlo.