Email Vs. Direct Mail

The cost per.. is the cost of the printing, having the address' and sending them.. not to hard to figure out.. you can google this expense. ( https://www.everydoordirectmail.com/pricing - 500 post cards at .33 each is $1650.00 )

Acquisition cost.. say you send 5000 cards... you sell 10 whatevers... the cost becomes the total cost of mailing divided by the number of sales made. ( usng the number above at acquisition would be $165.00 )

Success... a semi bench mark would be 3% or there abouts..meaning a 3% conversion rate. Send out 5000 cards and sell 150. ( if the goal is 3% and you convert 150 of the 5000 your acquisition cost would be $11.00 )

That will depend where in the world you are and the type of direct mail. If your client is already using a direct mail service, then they'd already have the answers... So ask them.

As for measuring success, if it makes a profit then it's a success*.

If you're really asking about tracking (rather than measuring success), then coupon codes, unique package names, special URLs where they can order, lots of ways to track response...

As an aside, there was a study done here in UK that concluded that the "real" experience of the physical piece in your hand would tend to invoke more emotion and be memorised better than the same piece sent digitally. Hence retain brand association and better motivation to buy. That's also been our experience, though with the caveat that it all depends on what you're sending...!

* You need to know your customer lifetime value. It's possible for a successful campaign to make a loss, if you have a backend process to market to the new customers and consequently make it profitable.