Postcards vary in value depending on the subject matter, age, publisher and medium.
In general, most postcards are worth $2 - $5 however, the postcards of most value on today's market are those with a Halloween theme. They can range from $40 - $400 each depending on the condition and graphic.
"Real Photo Postcards" or "RPPC" can also be of value depending on subject. You can tell an RPPC because it looks like a photograph but the back is printed as a postcard for mailing. RPPC's of value are those with Alaskan and California goldminers, American Indians, people with toys or teddy bears, cute animal photos, Shakers, Cowboys, Railroad depots and trains to name a few. Each of those can be worth $20 - $200.
I am just checking in to see if you have any additional questions about your item?