Orangeways operates a cheap bus service between Budapest and Krakow which takes (apparently) 6h30.

However, the schedule currently shows departures on Monday and Wednesday mornings, but not Tuesday (April 2nd). Also, booking is not open for April yet.

The departure is at 7am, but on the Hungarian version of the website, it says from December 1st, departure is at 6am.

Eurolines also operates a bus service with a similar schedule, but April departure dates have not yet been published.