I asked about the full one-way 2nd class fare Budapest - Vienna and it is €38. This is ridiculous, because you can buy an unrestricted, walk-up round-trip fare for €31 (valid on any train). This is valid for 4 days, and you simply don't use the return. Not only that, but you get free public transportation (metro, buses, trams, local trains) in Vienna for the first two days with this ticket!

Note this ticket is only valid from Budapest - there used to be a similar 4-day round-trip ticket Vienna to Budapest, but it was scrapped some years ago.