If you use a design program like Canva or you’ve purchased a template from Etsy, you can print your postcards with a vendor like VistaPrint, where pricing (depending on paper quality, thickness, and quantity) can be as low as 8 cents per postcard. Then you’ll need to calculate postage; postcard stamps are currently 48 cents a pop.
If you use a full-service provider, like ProspectsPLUS!, the pricing is higher, but they do all of the printing and mailing for you, and can even send out multiple mailers for a marketing drip campaign. With ProspectsPLUS!, standard postcards mailed first class cost between 74 cents and 96 cents per card.