Sleeping is the biggest concern after air-fare. Nothing is worse than coming back to a dingy, charmless hotel with no elevator or air-conditioning after a hard day's sightseeing...except maybe spending extra for a "nicer" hotel only to find that the elevator is broken, the air-conditioner isn't in your room, and the staff are downright rude. You've got to make a decision to either "go budget" and deal with the minimum facilities (a private room with a bathroom and a door that locks in a decent neighborhood), or do a bit of research to find a nicer place that isn't a rip-off. I'm always amazed at how many €250/night hotels are actually worse than €100/night ones!
The Low-Budget Option
Youth hostels are your first option, and there are quite a few in Paris. For those who are actually under 30, the nicest are in the Marais, Les Maisons Internationales de la Jeunesse et des Etudiants, MIJE.com. Those over 30 can try others with more relaxed rules, such as the Peace & Love (near Gare du Nord), Paris-hostels.com or the Young & Happy (Latin Quarter), youngandhappy.fr. If you're really cheap you can camp in the Bois de Boulogne on the western edge of Paris, €18-25 to pitch a tent, or rent a little chalet/mobile home for a group: www.mobilhome-paris.com. Super cheap hotels can be found for under €50/night, many don't have phones or toilets in the rooms (down the hall with the showers). I usually find these by simply walking by and seeing the rates posted in the window of non-descript hotels. The tourism office can help you find these as well, but only on the day of arrival (i.e.: just show up in the tourism office behind the Opera or in the commercial center under the Louvre, and they charge €5 to book you a room in your price range): parisbienvenue.com. Eurocheapo has been my favorite for a long time (eurocheapo.com). They list a lot of classic dives with plenty of personality that they've checked out themselves. Which brings me to an aside: soooooo many websites with hotel reviews are all copied from a central database, and offer visitors no real honest info. The only site besides Eurocheapo that reviews hotels is Tripadvisor.com, which I think is a great resource no matter what hotel you're staying in (and be sure to fill out your own evaluation afterwards).
