Central Europe Triangle Pass

You have to do the price comparisons yourself. Here are the options to look at: 1. Flying round trip to Prague, buying the Central Europe Triangle Pass (hereafter, the CET Pass to save typing) for transit between cities. 2. Flying round trip to Budapest, buying the CET Pass for transit. 3. Flying into Prague and out of Budapest, buying the CET pass for transit (has the advantage of saving time by not backtracking, and may not cost more than a roundtrip flight). If you do this, don't look at the price of two one-way flights (very expensive); instead, use the "multi-city" option on the website, to get what's called an "open jaw" fare. 4. Flying into Budapest and out of Prague, buying the CET pass for transit (sometimes open jaw flights are cheaper in one direction vs the other, for no clear reason).
5. With any of the flying options above (RT to Prague, RT to Budapest, open jaw with Prague first, and open jaw with Budapest first), you can just book rail tickets in advance, without a pass. I don't know about pricing on these routes, but if your dates are fixed and you can commit to non-refundable and non-changeable tickets, it's usually cheaper than a pass (sometimes MUCH MUCH cheaper). To get routes, use the Bahn website. To get prices and book tickets, use the website of the originating country. For Prague to Vienna or Budapest, use CD (Czech rail); for Vienna to Budapest or Prague, use OEBB (Austrian rail); and for Budapest to Vienna or Prague, use MAV (Hungarian rail). All of my links are to the English pages of these sites.

Thanks Harold - will use your advice & websites