I set it up on my computer then run down to Kinko's and copy it to my letterhead, it's faster and cheaper than printing them off on my inkjet. Besides my time, it costs about 40 cents per double-sided copy. It's sent out with the invoices for that month so the postage doesn't cost any extra.
I don't track the additional revenue. While it's a consideration, I view it more of a service to my clients and a way to keep them informed. It also cuts down on the phone calls and emails I get from customers as they are "in the loop".