Hi Claire,
I'm not sure about renting a room, I guess it all depends where -and with which family- you want to stay.
I'll try to ask around.
Life in dar is quite expensive, I imagine you could hardly do with less than 150 or 200,000 Tz Sh per months (about 150 to 200$). But that really is a minimum. A meal in a local restaurant will cost about 2$ to 3$, but if you want to eat western food it jumps to 10-15$.
Taxis are expensive (3 to 5$ a ride), bus dala-dala are not but not very practical.
Food in supermarkets is expensive (+30% compared to Europe), but relatively cheap in local markets.
Hope this helps,
Gilles