Iaşi is an afordable city, where prices can be lower than in other Romanian cities. 400 Euro is enough for a monthly decent living. Here are some expenses you might have to cover:

Accommodation may vary from 390 LEI/month in C5 Residence, Titu Maiorescu Campus, to 580 LEI/month in Gaudeamus Residence, Codrescu Campus (prices of 2015-2016 academic year).
Meals at the student cafeteria: 300 LEI/month (lunch) + 240 LEI/month (dinner)

If you prefer to go out and eat, your expenses will be higher. Anyway, you can save money by cooking your meals in the student residence.
A meal in a restaurant, consisting of an average pizza and a fresh juice will cost you around 35-40LEI.

Local transportation: 80 LEI on one public transport track (bus or tram)
Books and stationery: 100 LEI