I highly recommend the Hotel Ottocento, located between the Spanish Steps and Piazza Barberinni. Rates vary by season, but a double room can be booked for around 200 Euros (maybe a little less). Quiet hotel, great service. Room was very comfortable, beautiful new bathrom, great buffet breakfast (included). You can book on venere.com and other hotel reservation sites, but you can get a better price if you email them directly. At least, I think the price will be better--depending on what specials are currently being offered through internet sites.
Mary Beth