Seat reservations usually do cost money, some more, some less. See http://www.ricksteves.com/travel-tips/transportation/trains/reservations. The next question is whether reservations are required for your particular trip, or for every leg of a connecting route. DB train schedules online are the most complete and objective indication of when a train is "subject to compulsory reservation." Eurail has also made an app for pass holders based on this technology.
If you are using Rail Europe.com to book seat reservations and are getting a response "train tickets found," then you need to explore further. If your trip is soon, then it's possible that seat reservations could sell out, for instance for a TGV or Thalys train touching France. But you could also get that message or another error if one leg of the trip is by non-reserved regional train or if you have more connections than the shopping cart can handle. Use DB to get schedules and break up your requests to reserve just the legs that require it, entering them separately into the shopping cart, if necessary. I would not book ahead for any optional reservations; these are not required, not likely to sell out, and are a bit cheaper at European train stations (e.g. $5 instead of $11).
Very few daytime seat reservations cost $70, but that could be the case on Thalys from Paris to Amsterdam in 1st class (you can choose 2nd class for $40) or from Paris to a Swiss city on a direct TGV Lyria in 1st class (you can choose 2nd class or choose a TGV to Strasbourg with an extra connection and $11 seat reservation). Private double sleepers on overnight trains in western Europe will typically cost $70 per person, similar to a hotel room.