When Aimen Barma was VP Marketing for Audible, she discovered that referral marketing was the cheapest of all her new subscriber acquisition channels. Cheaper than PPC, co-registration, online ads, rented lists, affiliates … in fact, her financial analysis showed referral acquisitions were 60% cheaper than any other paid media.This month I asked her to come present to our Subscription Site Insider Members all the details of what works in referral marketing. (An on-demand replay of this webinar is available here. ) As she pulled together her presentation, there was one big problem: aside from a very few sites she already knew about, she couldn’t find any more examples of referral marketing campaigns in action for digital subscriptions or membership sites.Which is nuts! We started looking. Slim pickings. Very slim.We are all trying to make our marketing budgets go a lot further these days. So, why are so many subscription and membership sites happily paying for PPC campaigns and social media ads such as facebook and LinkedIn ads…. when they haven’t even put a basic referral marketing program in place first? No, sticking a lame “social media icons app” on your public pages doesn’t count.This is not rocket science. I’m glad Aimen’s gotten me thinking about it for our own sites. I hope she inspires you too.