The price of point-to-point tickets can depend on how early you buy them, locking in travel dates and times. For an easy overview of regular fares, see Rick's Train Cost & Time Map. London-Paris always requires a separate ticket, which should be bought early for best price.
With a rail pass, booking ahead for the Paris-Germany leg is also important, since the faster French trains restrict the number of places for pass holders. If you travel Paris-Koeln on a Thalys-brand train via Belgium, the seat reservation with a pass that includes Belgium is about $50. From Paris to another change point near the German border (Stuttgart, Kaiserslautern) has a more normal seat reservation fee of about $14 and doesn't cross another country. Other German and Swiss day trains don't need reservations and Italian reservations can be made locally on short notice.
If you're going for a 4-country Select pass covering France, Germany, Switzerland, and Italy, then it is not valid for Thalys trains across Belgium (so choose the other route) and you'd also have to buy a ticket for any small incursion you might make into Austria. Cost for a youth pass is currently $404 per person for 6 days or $471 for 8 days of travel within 2 months. A pass also covers K-D Rhine and Mosel boats and Swiss lake boats on the same days that you use for trains; and gives you 25% discounts on many Swiss mountain lifts (without using a travel day).
For trains within Germany, you can book sleepers through the usual DB schedule link and during the process, select "book only extra charge."