Overnight train Prague to Budapest

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I'm sure this question has been asked, I just can't find it.  First time in Prague March 23 AM arrival.  Two of us leave for Budapest late evening on the 26 th, arriving in Budapest early AM the 27th, returning to Prague late evening of the 29 th.  I'm looking at the overnight train, and want a 2 person sleeper if possible.  Is it better to buy my tickets when I arrive in Prague which only leaves a 3 day window or try to book online?  And, should I buy both departure and return tickets at the same time?  I'm not sure I can do that.  Does anyone know what I will expect to pay for these tickets?  Thanks for your help in advance, greatly appreciated.

Does anyone know what I will expect to pay for these tickets?

One way full fare for connection Prague-Budapest is 57.2 EUR, while return CityStar ticket is 100 EUR for the first passengers and only 50 EUR for the second, thus two passengers return ticket will cost you 150 EUR in total (roughly 75 EUR each) 
Sleeper reservation for "double" will cost from 21 EUR to 39 EUR per person (depending on carriage - 21 EUR for group of carriages going Praha-Budapest and 39 EUR for group going Berlin-Praha-Budapest). Unfortunately ticket for the overnight train can't be purchased online (for daytime trains you actually can buy ticket online, those start from 19 EUR).
In total for the whole return trip each will pay 75 EUR+21/39 EUR*2

If you want to purchase tickets online, then you might consider purchase of tickets to Budapest from German station, for instance Dresden. The thing is that overnight train from Prague to Budapest is nothing else than transit train EN 477 Berlin-Budapest. In Prague just couple carriages added to it, thus you can buy ticket from German Railways website (http://reiseauskunft.bahn.de/bin/query.exe/en?newrequest=yes&protocol=http:&) for route Dresden-Budapest, but board the train in Prague. It is fully legal, so no problem with late boarding. As of know, for March 26th DB offers tickets Dresden-Budapest for "double" for only 69 EUR (ticket+sleeper reservation in one), same story with return tickets Budapest-Dresden.
In total for the whole return trip each will pay 138 EUR (or 69+69 EUR)