Purchasing train ticket

I trust thetrainline.com. If you want to provide the specific itinerary, perhaps you can
get more detailed help. Otherwise, I do think it's a good idea to compare fares across
more than one platform. Note, for instance, that the Swiss rail site assumes you have a
Swiss Half-price fare card (which most Swiss people who travel a lot have). Something
like that can obviously potentially account for a price difference.

How far out were you looking on the Oebb site? When I looked at midweek next week, prices were around €108 for midmorning. The same midweek prices 3 months out were around €25. I always avoid purchasing on 3rd party sites. They're fine until you have any kind of problem. Then good luck. Buying in advance, when you are sure of your dates and times, will usually save money. But they may be nonrefundable and non-transferable. Always read the T&Cs.

Who's operating the train?

I went from Munich to Vienna last month and booked on the OBB website because they were operating the train.

Never buy tickets from a third party reseller.

For border crossing trains it is advisable to check the fares in either country.
Austria: https://www.oebb.at/en/
Germany: https://int.bahn.de/en

Thank you for all of the advice!