Hi Tim,
The thing with Hungarian Railways (MAV) is that they start tickets sale (including those tickets sold online via MAV website) only 60 days in advance, so you obviously can't see (and can't purchase) tickets & prices for August. If you check earlier dates inside the ticket sale range, then you'll get an idea.
To simplify, in case of Budapest to Salzburg you have three relatively cheap options: a) purchase in advance discounted 29 euro tickets for direct daytime Rail Jet trains; b) don't bother with any ticket purchase in advance and buy your tickets already in Budapest form the ticket counter, here you'll need so-called "Excursion ticket" (Kirándulójegy in Hungarian), those are 39 euro per person (you can buy them as well via MAV website, but price at the ticket counter is the same, so no need to bother with MAV online ticket shop) and finally c) purchase two separate tickets - first one discounted one-way from Budapest to Vienna (13 euro, via MAV website), while second (can buy it for the same Budapest-Vienna-Salzburg train) from Vienna to Salzburg (from 19 euro, via Austrian Railways website). Guide on how to purchase Hungarian train tickets -
http://europetrainsguide.com/Advice/How-to-Buy-Train-Tickets/Hungary-MAV.html , Austrian train tickets -
http://europetrainsguide.com/Advice/How-to-Buy-Train-Tickets/Austria-OeBB.htmlIf you have further questions or can't find something - let us know, we always here to help