Salzburg to Budapest train tickets.

OEBB is not a vendor, but the national railway company who runs this train. Prices for advance-bought tickets for this date vary between EUR 24 and 39, depending on the time of day. Some connections involve some changes. I'd go for the direct Railjet connection. Tickets can be bought at their website.

Scott, The first place you should look for train tickets is always the company running the trains.
ÖBB = Österreichische Bundesbahnen = Austrian Federal Railways = http://www.oebb.at/en/
I would never search Google for buying tickets, I would always go directly to the train company by name.
Some resellers give the same prices as the train operator's site, and cover more than one country (Loco2, Trainline), but others, especially the ones outside Europe, have price markups and only offer some trains.

From Salzburg to Budapest you will have to change in Vienna. For info on Salzburg to Vienna see here: http://www.seat61.com/trains-and-routes/vienna-to-salzburg-by-train.htm
For Vienna to Budapest see: http://www.seat61.com/international-trains/trains-from-Vienna.htm#Vienna-Budapest

you will have to change in Vienna
Some connections - such as the 09.08 - are direct, without a change.

Yes, there is a lot of ticket price variation on most faster trains in western and central Europe. That's mostly based on the difference between advance-purchase discounts and full fares (more than agent mark-up on any particular fare). And not all agents can sell the advance rates. So be aware that the price of a 2nd class ticket goes up to €92 from OBB if you wait to purchase at the last minute.

OK, thanks. To get the discount on the tickets how far in advance should I purchase them?

For April? Now.
And unless you are travelling at an off-peak time, you really should pony up for a seat reservation.

The system is showing me a Sparschiene tickets for €39 on the direct rout departing at 9:08. Those will not last.

Thanks, I booked the 9:08 direct to Budapest @ 39 + 6 for the reservation. Nice !

Well done!