It depends on what sort of trip you're looking at. Are you looking for a relaxing vacation in fancy hotels and nice restaurants? Or are you looking at getting better value by staying at cheaper places and being able to see more sights?
Paris has a very large range of hotels - that range from very cheap (Hostel Prices) - which results in places that are smaller than most motels in the states, and just about as nice - to very lavish hotels.
Easiest way to get an approximation of prices - is just to Expedia it - you get a range of hotels (maybe not the best deals for those specific hotels), but you'll be able to see where they are around the city.
I personally stayed at Hotel Du Commerce a few years back - then stayed at a few random hostels and a few crappy cheap hotels in random places.
I liked the hotel because it was in a quieter location right by the Latin Quarter... The rooms are really spartan though, so if you're looking for something nicer, look elsewhere.
http://www.commerceparishotel.com/
London - you can find cheaper hotels, but again, depends on where you want to stay and how much.