I travelled around Belgium, France and Spain with Interrail in 2001 and I remember that I was really supprised by the extra fee on the TGV's in France - it was less than I expected. The extra fee for Paris to Marseille was less than 5 Euro for example. But, travelling by train in Holland isn't cheap, neither are the extra fees. Compared to the TGV's in France I had to pay much more for the Thalys from Amsterdam to Paris (just checked it - that was an extra fee of about 20 Euro). I don't really know if there's a standard rating for the fees though...
As far as I know you only have to pay extra in Holland on the Thalys from Amsterdam to Brussels & Paris and the ICE to Germany. All other trains are free with Interrail (unless you'd travel First Class, but you don't want to do that). I believe there are no more overnight trains leaving from any city in Holland to other countries, so there's no problem with extra fees on those... In other countries you usually have to pay extra for overnight trains if you book a couchette or whatever kind of sleeping place there's available - normal seatings in overnight trains usually don't cost you anything extra.

I hope this helps you a little - I know I searched a lot two years ago on the WWW for the extra costs, but couldn't find them anywhere. It's kind of hard to tell what the costs are exactly and the prices I just mentioned are from the pre-Euro era, I guess they changes quite a bit as well.