When the question on Budapest (really Hungary) first came up I did a bit of research and discovered that it has been a country where tipping is the norm going back at least 100 years. I actually found a reprint of a letter written at the turn of the 19th century from a tourist to his son warning, or advising, that the Hungarians expected tips and how much to give. Sorry, I just love the history and the research......
Anyway the recommendation in the letter is the same as now. Usually just round up the charges and aim for about 10% as a minimum. For Taxis a few hundred will suffice.
There is one difference between Hungary and the US in that there isnt the same social pressure to tip. If you do, the general feeling is they appreciate you. If you dont the general feeling is that they have no animosity. That might be because its a tipping culture sitting in the center of a continenet where tipping isnt common and they understand that outsiders may not understand Hungary? Dont know, just guessing.
To make the thought process easier, if you are going anytime soon, the exchange rate is roughly where 1000 forints = $3.00. So, 10,XXX forints is 3 x 10 = $30