We'll be arriving in London from the states and then taking the Eurostar to Paris the next morning, so would like to find an inexpensive B&B ($100 or less) near Waterloo station. Any recommendations?

Waterloo is not an area w/ much in the way of B&Bs -- not even that many hotels.

How early is your train? - because you can get from anywhere in central London to waterloo easily by tube/bus.

The nearest moderately priced place is County Hall Travel Inn but it runs a good deal more than your budget. $100 is only about £55.

If it were me I would use Priceline for a full service hotel which can be won for less than $100. Check biddingfortravel.com for PL bidding strategy and info about which London hotels are being won in different zones.

check out The Mad Hatter in Stamford Street, SE1

The Mad Hatter is fine - but I'm pretty sure it runs nearly double vanne's budget. It is at least £100 per night and probably more.

Unfortunately this is probably well above your budget, but, if you can find a deal, there is a sofitel within walking distance. I stayed there before a morning Eurostar as well, and simply pulled my little black wheeled suitcase over to Waterloo in maybe 10 minues.