First of all, forget about pass, because Eurail pass is absolutely useless for such type of a trip, since you can do the whole journey in less than 70 euro if you purchase your tickets smartly.
And second,
Actually you can't purchase Budapest-Prague ticket via CD eShop unless you started your trip in Prague (Prague-Budapest-Prague), because CD conductor stamp needed on your return ticket to be valid. So the only chance to obtain 19 euro Budapest-Prague ticket is to purchase it in advance (3 days) at any international counter in Budapest.

p.s. you can as well look at http://europetrainsguide.com/ for some ideas on budget train travel in the region