I get small quantities printed by Vista Print. I usually get a double sided print. On one side I put a generic promo flyer ( either for garments or for signs). On the other side I put a more targeted design for various market segments - schools, workwear, real estate etc.
I tend to send out 100 every other week, with a business card and covering letter. Works out at about £1 ($1.50?) per envelope. I only need one reply per batch to make it worthwhile.
I think using a targeted design is more likely to stay on someones desk than a 'spray and pray' on size fits all approach.