Ah OK, sorry I didn't realise that.
So lets say, worse case your rent i s 12000 per year (but will prob be cheaper) they'll pay 10,000zl, now it's looking much better.
You're pretty much rent free!
That would make a huge difference.
Still though be careful, go through the contract with a fine tooth comb and insist they you're given it in Polish and Dutch (or at least English - so you can get it cheaply translated into Dutch)
If.....and only if you're living where you're rent will be reimbursed, then you'll enjoy living here a bit more. However, you won't save much 2900 isn't a great monthly salary.
According to 2010 figures, the average salary in Poland is around 3400zl. But it depends on where you live. In Warsaw it would be higher (4800). In fact the highest is in the south in Katowice (around 5000zl)
So as you can see 2900zl isn't wonderful, even if they do pay your rent. It's not minimum wage, but it's still well below the average.
If I was you I'd demand more.