I have found an 800 number to be effective from the image/branding side. People assume its expensive (it isn't: my bill is $15 a month), and it projects an image of success/credibility/stability. It's one of the little things that says "this guy knows what he's doing" that make the sale easier. So it has been worth it to me on that end.
The same thing is true of reply cards/SASE's. It isn't about the .42 cents they save, its about the convenience to them of being able to just put it in the mail box. Its one of the little details that shows them that you are focused on making their life easier, focused on their needs.
Now, if you are doing direct mail with reply cards; the postage on the reply dramatically raises the cost of the whole mailing. You would have to get, in my view, better than twice the response rate to justify the cost (because you could be using that same postage to reach twice the number of prospects, which would equal twice the response rate) But for contracts and and testimonial letter requests, I've always provided postage.