The short answer
Five hundred 4×6 postcards, printed and mailed First Class, costs $595 on CardCadence. That is 500 × $1.19, and the price includes printing, postage, and the mailing itself.
A 6×9 card puts the same drop at $675, and a 6×11 at $845. There is no setup fee, no monthly platform fee, and no minimum order, so the number you calculate is the number you pay.
That is the easy part of the question. The harder part is whether 500 is the right number, and whether those 500 addresses are the right ones. Those two decisions move your return far more than the difference between card sizes.
Where quotes usually hide the cost
Printers quote you per piece and stop there. That is fine if you are picking up a box of cards, but a mailed campaign has three more costs, and only some vendors show them.
Postage is the big one. First Class postage on a postcard is a real per-piece cost, and a quote that shows you a printing price without it is not a quote for mailing — it is a quote for printing. Ask which one you are looking at.
Then there is the platform fee. Several direct mail tools charge a monthly subscription on top of per-piece costs, often a few hundred dollars a month. If you mail once a quarter, that fee can cost more across the year than the cards themselves.
The third is list cost, which we will come back to, because it is where the real money is either made or wasted.
The arithmetic that actually matters
Stop thinking about the cost of the drop and start thinking about the cost per new customer.
Say your 500 cards cost $595. If two percent of recipients respond, that is 10 responses. If half of those become customers, you have 5 new customers for $595 — about $119 each.
Whether that is good depends entirely on what a customer is worth to you. For a business where a first visit is worth $80 and a customer never returns, $119 is a loss. For a dental practice where a new patient is worth several thousand dollars over the relationship, it is one of the cheapest acquisition channels available.
Work out your own number before you mail anything. If you do not know roughly what a customer is worth to you, that is the thing to figure out first — not the postcard.
Why the list decides the outcome
Two businesses can mail the identical card, at the identical cost, and get results that differ by an order of magnitude. The difference is almost always who received it.
Five hundred cards to people who have already bought from you and drifted away will outperform five hundred cards to strangers, usually by a wide margin. They know your name, they have been through your door, and the message can reference something real.
This is why a small, well-chosen list beats a large, cheap one. If you have 300 lapsed customers, mail those 300. Do not pad the list to 500 with people who have never heard of you just because the round number feels better.
A sensible first drop
If this is your first campaign, mail your own customer list before you buy anyone else's. It is the cheapest list you will ever have, it is the most likely to respond, and it tells you whether your offer works before you spend money finding new people.
Keep the first drop small enough that a failure is survivable and large enough that the result means something. A few hundred cards is usually the right range: enough that a two percent response is a handful of real responses rather than statistical noise, and cheap enough that you can afford to learn from it.
Then measure. A QR code on the card turns a vague sense that things picked up into a countable number, and that number is what tells you whether to mail again.