For example, I've gotten multiple 1 month free trials, and also the '1 week for 1.99' which you can immediately cancel after placing your order for a $1 refund (if you immediately cancel, not at end of the week).
I don't shop at Amazon often, but I've gotten many (read: > 10) Prime shipments over the last year, probably paying a grand total of $3 or so.
If I don't have any good Prime offers upon checkout, I'll go to Ebay, find the product, soft by lowest price, and it's usually the same price, or within a dollar or so, and ships a LOT faster than Amazon.com's standard shipping (usually ends up coming from Amazon anyways proxied by the Ebay seller with Prime).
I've gone through the Amazon cancellation pages many times. They always keep tweaking it and expanding the process.