Hi again.
Calais to Paris Nord will, as stated above cost you �35.50. That assumes that you use the 'sensible' service at 0643, via Lille. (Remember that the journey involves using a TGV service which requires pre-booking. Use the SNCF link above).
The whole of the RER journey, from Gare du Nord to Champ de Mar - Tour Eiffel, is within Zone 1 of the Paris transport system, so the single fare is just �1.50.
If you're planning on further travel within Paris, you might also want to consider these tickets:
Carnet: This is a book of 10 tickets for individual journeys within central Paris. There's no time limit for using the tickets. The carnet costs �11.10
Paris Visite: There are two versions of this tourist ticket. One covers 6 zones, the other covers 3 zones. You almost certainly don't need the 6 zone version, so I'll just give the 3 zone prices. A 1 day ticket (for unlimited travel on Metro, RER and bus services in Zones 1 to 3) costs �8.50. The 2 day version costs �14. 3 days costs �19. 5 days costs �27.50
However, most tourists don't actually need to travel as far out as Zone 3. It's cheaper, for one day's unlimited travel, to buy a 2 Zone Mobilis ticket for �5.60.
Chris
PS: I used to run a major railway station, so sorting out rail information is second nature to me. As I rail employee, I also got cheap travel on Eurostar - just �19 for any journey - so I got to know Paris quite well ;-)