Amazon’s original content efforts are paying off, at least in terms of driving new sign-ups for Prime, which is ostensibly their main value to the commerce giant. Documents obtained by Reuters show that the top Prime video original programming, including 19 shows covered within, were accountable for as many as a quarter of the total Prime service sign-ups between the end of 2014 and the beginning of last year.

The total U.S. audience for Prime video, including original and licensed content, was around 26 million, according to the documents, and in total around 5 million new subscribers joined Prime specifically for programming, including 1.15 million who signed up for Amazon’s The Man in the High Castle series alone, per the documents. Figuring cost of acquisition based on the $72 million production budget of the show, Amazon’s internal papers show that each $99 yearly subscriber cost them a one-time fee of $63 on average for that group of new members, which helps explain why Amazon is so keen on building out its original programming slate.

But how does Amazon know that a show led to a Prime sign-up? They track what a Prime customer streams first after becoming a member, and though the report isn’t specific