The short answer is that it costs 34 cents to mail a first class postcard.
Yet, if postcards are presorted then they can apply for bulk mailing rates ranging from 25.3 cents to 28.4 cents apiece depending on the distribution of the mailing list. Postcards are only eligible for first class mailing rates, and thus can only be presorted if there is a minimum of 500 postcards. The USPS also has a very specific definition of what a postcard is. The USPS defines as postcard as having a maximum height of 4.25 inches and a maximum length of 6 inches.
Many mail agencies prefer to mail larger “postcards” (for example 9 inches by 6 inches), which the USPS classifies as letters and we will refer to as “large postcards”. Some advantages of mailing “large postcards” is that these will stick out in the mail, be more noticeable, and provide more opportunities to add cover or images. These “large postcards” also qualify for marketing mail postage rates which makes the cost of mailing comparable and sometimes even cheaper than mailing small postcards at first class rates. The cost of mailing a “large postcard” at marketing mail letter rates ranges from 28.8 cents apiece to as low as 15.5 cents apiece. Additionally non-profits are only eligible to use their non-profit rates on marketing mail so first class small postcards are ineligible to be mailed at non-profit rates. Non profit mailing rates range from 17.1 cents apiece to 8 cents apiece.
Many other factors may affect the price of postage on a postcard including the quantity mailed, distribution of the mailing list, weight, dimensions, and thickness. It is best to discuss and plan your postcard with a mailing house in order to get the most bang for your buck out of your postage.