Paris-Munich can be rather easily done by daytime high-speed trains (ICE, TGV) or overnight CNL trains. Tickets should be bought some time in advance, that assures the possibility to get discounted tickets (starting from 39 euro). Can be bought from DB website.
Munich-Vienna: many trains per day, DB Europa Spezial tickets for as low as 29 euro.
Vienna-Budapest: same thing, trains go every 2 hours during the day, tickets start from 19 euro (
https://ticket.oebb.at/bin/frame_ticket.pl?ticket=jticket.pl&LANG=EN)
Budapest-Bucharest: a number of options in terms of timetable depending on your preferred time of departure/arrival. If you will stay in Budapest more than 3 days, then you can purchase discounted "Fortuna" ticket - from 29 euro. No ways to purchase online, so you will have to do it from MAV counter.
Bucharest-Istanbul: it seems that you have two major option here: a) go with regular ticket (app. 40 euro + 10 euro for couchette or 20 euro for 3 bed sleeper); b) if you intend to do some additional travels around Balkans, then it is better to consider Balkan Flexi pass (5 or 10 days within 1 month)
In case of Italy to France rail travel you should hunt for one of the TrenItalia special offers -
http://www.trenitalia.com/cms/v/index.jsp?vgnextoid=9bd599e8e609a110VgnVCM1000003f16f90aRCRD