Mark - Have you ever tried a bulk mail stamp? Most people would NEVER know its not a first class stamp on the envelope.
Brooks - setting up the account at the post office is pretty simple, though not exactly cheap. If the post office does bulk mail (most do), they have a person assigned to this. They will help you fill out the form and help you with your first few mailings. You go in the back door of the post office for bulk mail, so it's much, much easier.
It depends what you mail, how you sort it and what type of post office you have.
In general, last time I did the calculation about a year ago, first class postage was $0.44, I paid $0.262 per piece. You can get a bit lower than that if you mail enough or drop them off at a larger processing center, but this is a decent, realistic example.
At an $0.18 savings per piece, if you only mail maybe 2000 pieces a year, it may not be worth it. On the surface, you would save around $360.00, but you have your permit, which I think is $150 (or is it $180?) and then the hassle of mailing them out bulk mail, meaning they have to be sorted in zip code order, etc. On the other hand, if you mail 2000 a month, it would be worth it.
In either case, it's probably cheaper (and faster) to have a mail house send them for you.