Since you're looking to barter for printing services, the company your approaching will probably have their own prices advertised already, which simplifies one half of the discussion.

As for your end, if you can point to what services similar to yours cost elsewhere in the open market, point to that.

You can also ask your prospective client what they currently pay for such services. I'm guessing a printing company will have paid for graphic design in the past. So they ought to have numbers on file for both sides of the transaction.