Values of Collectible Postcards are determined by the age, type, and subject of the card.  Here's how to determine the age:

PIONEER ERA (1893-1898)

This era began in this country with postcards put on sale by vendors and exhibitors at the Columbia Exposition in Chicago in 1893.  These proved to be enormously popular, but are now scarce.  They can be identified by a combination of the following:

Undivided backs

Most will be multiple view cards

Many say “souvenir of” or Greeting From”

The most common titles are “Souvenir Card”, or “Mail Card”

Postage rate is 2 cents

None show the “Authorized by Act of Congress” byline. 

A Columbia Exposition card sells for between $2-6 depending on subject.

PRIVATE MAILING CARD ERA (1898-1901)

In 1898, the US government gave private printer permission to print and sell postcards. These cards were typically reprints of the earlier Pioneer cards, with the inscription “private Mailing Card” added.

UNDIVIDED BACK ERA (1901-1907)

These postcards have undivided backs intended for the address only.  Short messages were written on the front, or picture side of the card.  Used cards of this era with writing on the picture side are acceptable by collectors

DIVIDED BACK ERA (19071915)

The divided back provided space for the address and the message on the back of the card, and left the front unmarked.  Collectors loved these cards for mailing scrapbooks and trading.  Typically, the image filled the entire front of the card.

WHITE BORDER ERA (1915-1930)

This era began when postcards ceased being imported from Germany during the First World War.  US printers began producing postcards to fill the void, but these were of lesser quality than their German predecessors.  The White Border Era signaled the end of the “Golden Age of Postcards”.  Postcards of this age were often reprints of earlier divided back era cards with a white boarder around the image.

LINEN ERA (1930-1945)

Postcard manufacturers began using higher quality paper with a high rag content to produce cards with a textured surface, but still used inferior inks. Linen era cards depicting Roadside America, Black Americana, comics, and advertising are very popular with collectors.  These subjects can bring $10-15..  Typical tourist area views are worth only a few dollars.

PHOTOCHROME ERA (1939 TO PRESENT)

Introduced in 1939, photochrome cards became highly popular with collectors because of their beautiful colors.

REAL PHOTO POSTCARDS (1900-present)

These cards often have the photo processing information in the stamp box, and can sometimes be dated based on that.  With some exceptions, these cards bring the highest sums, especially if the location of the photo is indicated. Depending on the subject, these cards range in price from a few dollars to $50.  High prices are usually for cards with cross-over appeal, that is, subject matter that appeals to many categories of collectors.

Read more about Postcard Values Here.