As of July 2011, the rate is $0.29.
To mail a first class international postcard to Canada, you will need to attach $1.10 in postage. The maximum size for a postcard is 6" in length x 4.25" in height.
Currently, 34 cents.
As of right now (August 2008), postage for a postcard from Quito, Ecuador to the US is $1.56
44 cents
postage for a postcard sent from Australia to the U.S. costs AU$1.40.
98 cents
USPS.com says it costs 98 cents to send a postcard from the US to Australia.
The postage rate for a standard postcard in the US is 32 cents.
For a postcard that is 6 inches long by 4 and 1/4 inches high, first class postage in the US is 27 cents as of June 2008.
As of May 2011 any postcard over 4.25"x6" and less than 6.13 x 11.5" is considered regular First Class and is $0.44
postcard is .98 cents
From US to Netherlands it is 94 cents.. (From the USPS website)