No need to worry about the weather. In June, the day time will be a bit hot and in the morning and at night, it will be a bit cool but not very cold indeed. Just a thin jacket will be enough.
Among the three cities, Vienna will be the most expensive and most hotel will be over 100 euro. Budapest will be cheaper but nowadays is not as cheap as before.
Budapest is a beautiful city and I like the most. You should also try the hot spring there. Another city near Prague (Karlovy Vary) also famous in hot spring and you can try also.
If you are not prepared to spend lot of money in shopping, I think 1000 euro is enough. (not include accommodation fee)
Since you have 15 days, I suggest you can go to Salzburg also.
* I have been to East Europe before, like Czech, Hungary, Austria, Croatia, Slovenia, etc.
2008-05-04 03:42:35 補充:
You can buy the adaptor in 實惠