If I were to eat fast food( McDonald's) everyday twice,
I've never eaten fast food in Poland, but have looked at prices out of curiosity. It's pretty much on a par with fast food prices in the UK, maybe a bit cheaper.
It's not expensive.
Why don't you try some Polish food while you're there? Eating McD's twice a day is a bit boring and not exactly healthy.
shop a bit ( zara, h&m),
Both Kraków and Wrocław have big shopping malls, but prices for branded clothes are similar to those of the west.
stay in a three star hotel( room will be shared with a friend),
You will easily find good, well priced accommodation in both cities.
As InWroclaw has commented, book in advance as availability will be more of a problem in the peak summer months.
Make sure you see the Panorama painting when you visit, it's well worth it.
Agreed. It costs about 25 pln and includes admission to the National Museum too.
Worth noting as well that many museums have free entry days, and even if you have to pay for admission, prices are very low.
If you don't fancy a trip to Auschwitz when you visit Kraków, do try and visit the Wieliczka salt mines. Well worth a visit, not expensive, and about 10 kms outside Kraków. Most decent hotels will have information on these trips.
In addition, try and pick up copies of Kraków/Wrocław InYourPocket guides. They are free from tourist information centres/some hotels, and probably to download too. Lots of info in them about restaurants, clubs etc.
Minus accommodation, let's assume I'll be left with 300 pln per day for food( american fast food chains), clubbing( one or two drinks), sightseeing etc. Will it be enough?
That amount should be ample I would think, but then it depends on what you're drinking in clubs and whether you just stick with the one or two ;)
Sure you'll have a great time, both are lovely cities :)