Well... if you buy an iMac in the US and have it shipped to Europe (which you will have to do on your own) the cost will be aout $80 (I'm guessing, here)
Also, you need to take into consideration that Europe uses a different power current than in most of North America, so therefore you may need to get some of converter or NA-->European plug changer.
Lastly, I think the best decision would be to buy the iMac from one of the Apple Europe stores (which country will you be in??) and have it shipped from them, for free, to the place where you will be staying.
If you dont want to do that I would reccommend an iBook or PowerBook instead... much less hassle.