What's the cheapest way by train from Salzburg to Vienna?

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I know that in the other direction, from Vienna to Salzburg, you can avoid paying the 47,50 euros domestic fare by taking advantage of the Europa-Spezial Vienna-Munich discounted fares from as little as 29 euros and getting off at Salzburg. However, I'm travelling from Salzburg to Vienna. And I don't think it's possible to buy a Munich-Vienna special fare ticket but get on the train at Salzburg. Is there any other way to avoid paying the exorbitant 47,5 euros fare from Salzburg to Vienna?

And I don't think it's possible to buy a Munich-Vienna special fare ticket but get on the train at Salzburg.

Why not? You get ticket for a certain route, pay the fare (in your case discounted), afterwords it doesn't really matter whether you travel full route or only part of it (many explanations here, for instance - change of circumstances or your schedule).
Plus official Europa-Spezial "terms of the offer" (http://www.bahn.de/i/view/GBR/en/prices/europe/europa-spezial.shtml) doesn't have any specific restriction on the use of the ticket only from starting to the final destination. In most cases Austrian conductors won't even know from where exactly you travel from. 
So don't worry and purchase Europa-Spezial ticket from German station till Vienna for 29 euro.
Another option - purchase disocunted Salzburg-Budapest ticket from OEBB website (https://ticket.oebb.at/bin/frame_ticket.pl?ticket=jticket.pl&LANG=EN) for 39 euro and get off in Vienna.
Slower, but cheaper alternative - Einfach-Raus-Ticket for 28 euro (changes of trains required)