A good hotel that is properly central in Rome can be quiet expensive. We rented a beautifull apartment in a 15th centuary building that was a 100 metres from the Pantheon that was 100 Euro per night in june. Loads of sites doing them and would recommend it.
have stayed in the Hotel Piemonte. it is a 2 minute walk from Termini train Station.
it is advertised at €130 approx a night but I'm sure you could get it cheaper with some of the booking agents.
From my visit there, two general recommendations, don't go for a 3-star hotel (because they are nothing like that) and don't get a hotel near Termini. This one has come up a few times, I did a search on Google using the advanced search and limiting my search to Askaboutmoney.com and I came up with a few results.
We stayed in hoel Dei Mellini and can highly recommend it. We stayed in Jan 2005. We walked everywhere in Rome and although some places were a long walk it was very doable with good shoes!