paris metro

If you only want to know the metro fare. It is not that difficult. RER is more complicated.
The standard ticket is ticket "t+". It is valid for 90 minutes and for one continuous journey. It can be used on the whole Mtro network, on buses, trams and on zone 1 of the RER. The ticket allows unlimited transfers between the same transport (i.e. Mtro to Mtro, bus to bus and tram to tram), between bus and tram, and between metro and RER zone 1. It can be bought singly at €1.50 or in tens, a carnet, at €11.10.

圖片參考：http://www.ratp.info/picts/touristes/photos/tickett_1.jpg

But if you are travelling outside zone 1 by RER, then you have to pay according to your destinations (fares are not calculated simply based on the zone where the destinations belong).
To get the rates of RER, use the following website:
http://www.transport-idf.com/frontal;jsessionid=B81C007C69B74EABB0EF1C7CB430F430.tomcat?controller=Default&langue=en
Just type the name of the places and it will show you all routing alternatives and the time required. Once the routing is shown, click for details and then click for the rate. The rates for all the connections will be shown. I strongly recommend you to use this website.
Back to your question: If you buy one ticket each time, what is the cost of a single ticket from zone 1 to zone 3/4?Which one is cheaper?
The answers are:
If you are travelling on metros:
Forget about the zoning, all you need is a t+ ticket which costs you €1.50 per single purchase or €1.11 on average if you buy un carnet (10 tickets at one time).
If you are travelling on RER:
It depends on your destination. And unless you are using the Paris Visite (day-passes) or other passes, you have to buy your ticket every time. Check the rate with the website above.
You may consider buying the 1-day, 2-day or 3-day Paris Visite. Paris Visites are available in zone1-3 and zone 1-6.
http://www.ratp.fr/

It will be a bit difficult for searching the fares of tickets from Paris to Banlieue (Zone 3 or further). You can follow steps below:

Visit the website www.ratp.fr (although it's French, no problem)
Cilck and choose "Titres et Tarifs" on the bar on the top of screen
Choose "Rechercher un Tarif" and choose "Billet" on the first box and "Billet Ile de France" on the second box
You can see a box in the middle "Saisissez votre dare de Banlieue" Type the name of station and choose from the list, the click "Rechercher"
Then you can see the fare "Billet l'unit" means single ticket. "Plein tarif" means full fare.
If you don;t understand what I mean, you can send message to me.