Whilst I do not know your dates, I just did a quick search for 5n hotels in Rome on eBookers for Sep and I came up with several good hotels that were under �500 for the 5 nights. Some were quite significantly under �500 as well - such as the Augusta Lucilla Palace, or the Eurostars International Palace. Xx Settembre was only �374.30 for the duration. I do suggest that you read the reviews on tripadvisor.com before you make your selection.
Looks like the prices have gone up a bit though (or I guess pound is just weaker strong now)
It was very nice though, staff were brilliant, rooms not very big but clean and breakfast was really nice every day.
Near enough to underground station and the transport is very cheap.
I know you didn't ask for this but when you go to see the sights, go to a less busy one first and try and get an archeologia card (probably not correct spelling there!) it gets you into various places and you can skip the long long lines. You get a discount if you are under 25.
Tirreno Hotel, Elite Hotel, Express By Holiday Inn Rome-East, Champagne Palace Hotel etc. There are many hotels in Rome under $100. For more hotels check out this link Cheap Hotels in Rome
Hi Karen, We were in Rome for a 3 day break last September, while I wouldn't recomend our hotel as it was too far from the centre, I would say Rome is very expensive, we paid 24 eur for a large beer (split between the fellas) and two soft drinks, but I also would say, it is a very beautiful city too and perfect for a honeymoon, I wish you both well.