A much more difficult question than it seems….

http://reiseauskunft… The German rail website (best for schedules anywhere in Europe and prices in Germany) would normally show this price since the route ends in Germany, but it doesn’t in this case. Here’s the Polish rail website: http://www.pkp.pl/ Some pages have an English flag to switch to English, others don’t and I didn’t find prices.
If you’re planning on the overnight train, this will help: http://ourworld.comp… It’s last year’s prices, but prices rarely change and changes are usually very small.

Railsaver.com and railpass.com list the ticket price at $73 US. Raileurope.com lists it at $88. Given past experience, I would guess the price at about $55 US (plus you’ll need to buy a reservation for the overnight train, at least) — 30% less than railsaver/railpass/ the “bookings” tab here and 35% less than railreurope.com