Hotels.com has some of the best prices. You should expect about 70 Euro a night and up (esp. during high season).

Here are a couple of direct links to hotels to get you started (I don't know how good any of those are):

http://www.hotellaurentia.com
http://www.hotellapergola.com

Always compare the price found on Hotels.com to the price you could get by booking direct.

 

You should also try some cheap hostels, like:

http://www.hostelpinkfloyd.com/

At 14 euros/night and in the vicinity of the church Santa Maria Maggiore, Piazza della Repubblica and The Roman Forum, it's quite a bargain.

Thanks guys, I’ll try to find more cheap hostels, for a period of 5 days their prices are quite reasonable

I would look for hotels in province of Rome as well, since it's well connected with trains and they are much cheaper than the ones in the capital city. This will work if you won't mind travelling a bit for your hotel. Hope this helps!