...is better to get a rail pass or to get individual tickets from the country railway authorities.
That is one of the most essential & important questions many travelers ask, so let's just simply look at all your journeys and find the answer:
Berlin to Prague - 39 euro per person (bahn.com)
Website is correct, German Railways Ticket Shop is the place to buy such tickets, however discounted tickets purchased in advance start not from 39 euro, but from 29 euro per person. Ticket sale starts 92 days in advance
Prague to Budapest - ?? (cd.cz/en/ has timetables but I cant find prices)
Again, the website is correct - official Czech Railways (CD) page is the place to buy, you just need to go from general page to CD eShop -
cd.cz/eshop Tickets Prague to Budapest start from only 19 euro if bought in advance. Here you can actually travel overnight, won't give you a lot of time to sleep (00:01-8:35), but will save precious 7 hours during day time
Budapest to Salzburg - 39 euro per person (mavstart.hu / buy from ticket window)
Already discussed options in the other topic
Salzburg to Munich - 35 euro per person (internationalrail.com.au)
Forget about agencies like that, they just take advantage of inexperienced travelers and serve as a middle man, obviously charging their own commission. The thing you need for Salzburg to Munich journey is German group ticket for Bavaria - Bavaria Ticket (more about DB Lander Tickets on our
Germany page). Bavaria ticket is valid from Salzburg and sold in Salzburg, so you can buy it simply before departure - 22 euro for one person, 26 for two, 30 euro for three and up to 5 passengers, traveling with Bavaria Lander Ticket will be cheap and easy
Munich to Switzerland (Interlaken or Saas Fee) - ??
Here you have two options: a) easier will be just to buy in advance discounted ticket via mentioned above DB website, tickets like Munich to Interlaken start from 39 euro per person; b) more complicated and time consuming option will be again to take advantage from Bavaria Ticket - for instance use it from Munich to Lindau, then take a nice ferry ride over the lake Constance from there to Switzerland, while for Swiss part just get
SBB SuperSaver tickets, in total it will end up cheaper than 39 euro per person + more fun
Switzerland (Interlaken) to Venice - ?? (sbb.ch has timetables but I cant find prices)
Well, here it's not very easy to get the ticket and price you want, since there are no direct trains from Interlaken to Italy. Nearest direct train to Venice is from Brig (roughly an hour from Interlaken), while train to Milan is from Spiez (roughly 15-20 min. from Interlaken). Discounted tickets like Spiez to Milan start from only 27 CHF (app. 22 euro) and can be purchased via Swiss Railways (SBB) website, you'll need
SBB TicketShop and section "Trips in Europe" --> "The lowest fares for direct services to major cities in Europe"
Venice to Rome - 30 to 40 euro per person (trenitalia.com)
Purchase in advance via trenitalia website can get you tickets for as low as 19 euro person actually (or 9 euro when we speak about Milan to Venice in case you go Interlaken-Spiez-Mian), if you don't catch those, then next price level is 29 euro, these ones are much easier to get. Also besides Italian Railways (TrenItalia) the route from Venice to Rome is served by new train company
Italo, because of competition with TrenItalia their fares time to time might be better, so check both companies
...and additional fees for booking (it would be good to do some overnight trips, so I am assuming there would be additional fees for a sleeper?)
If you book via mentioned above official websites of railway companies (and not via various rip-off agencies) then you won't be charged any additional booking fees. Regarding sleeper reservations, the only journeys where you can count on more or less potential overnight trip is Prague to Budapest and Venice to Rome, in both cases reservation fares are rather mild (20-30 euro)
if I say on average each trip would cost 45EUR that is equivalent to approx 56AUD, therefore 7 trips, at this price, would work out better than the rail pass. But it would be good to know if this is a decent assumption?
Well, as you can see from the above, the average is even less than 45 euro, so in these terms pass is useless. However remember that almost all the point-to-point fares mentioned are discounted tickets requiring in advance purchase + with some exceptions they are nonexchangeable & nonrefundable, thus will save you money, but won't give much flexibility with your itinerary.