Can anyone recommend a bed and breakfast, centrally located, for no more than $150/night?
This article was in last month's Budget Travel issue - it has a few good listings for around $100.
http://www.budgettravel.com/bt-dyn/content/article/2008/05/26/AR2008052601313.html
Champs du Mar is 90 euros per night for a double. You have to book early as it is very popular--located in the 7arr--near the Eiffel Tower.
FYI, B and Bs aren't common in Paris. So if you are searching online, best to search for hotels that are 100E. I also recommend Champs du Mars but it is hard to get in. Logis du France is a very good website to check--they focus on good value lodging. You might also look at the Lonely Planet message board, since they are geared toward budget travelers. Rick has some other suggestions in his book but they sound like you get what you pay for.
Veronica,
Here are three highly rated budget priced hotels in the Latin Quarter...
http://www.tripadvisor.com/Hotel_Review-g187147-d567154-Reviews-Hotel_Diana-Paris_Ile_de_France.html