Hola:
I'm a Canadian who regularly travels to Cuba so can offer some advice. First off, the US doesn't care if your not an American or not, it where you reside that counts. You technically need an OFAC license to travel to Cuba if you're RESIDENT in the USA, regardless of nationality. However there are MANY Americans who visit Cuba via a 3rd country. Nassau, Cancun or Canada are usual gateways.

As to Casa Particulars, they are my preferred way to travel and vary from 20 CUC per night outside of Havana to say 35 CUC per night in a good Havana location. Now, that's a per/room price, not per person so if you're sharing a trip with someone your costs are lower. Most Casas also offer meals to their guests, typicall 3-5 CUC for breakfast and 8-10 CUC for dinners. And some of the Cuban moms, WOW, can they cook good. Truly a great way to explore Cuban cusine, not the tourist stuff in resorts or hotels.

As to booking a casa, it's easy to do on a day by day basis if your're travelling about. You'll find many casa owners at the local bus station with a sign and photos of their casa greeting bus arrivals. Also cab drivers will always know someone. And there is almost an unofficial "network" of casa owners who know other casas in each city. So if you're in one, just ask for a recommendation in the next city and chances are they'll call ahead for you.
You can also get lots of info and casa listings from another website specializing in Cuba travel.
http://www.cuba-junky.com/
At the top right of the main page you'll see a box for "Pototo" a resolver in Havana. He is a great guy and a personal friend who can help you arrange many great casas.

As to money, read this thread by my friend Terry (Martian) from another Cuba forum. Superb advice and everything you need to know to travel in Cuba.
http://www.tripadvisor.co.uk/Travel-g147270-c109329/Cuba:Caribbean:Money.Exchange.html

Anything else you need to know, just ask away.

Steve

Waoh, Steve ! Thank you so much.