As membership grows, cost per OA book goes down.
For example, if an annual membership of €800 (approx. $950 USD or £700 GBP) gives access to 50 backlist titles, and funds a further 25 OA frontlist books per year, that yields a total of 75 books per year that are accessible to your library users (and growing every year). With just 200 member libraries this would equate to around €11 (approx. $13 USD or £9 GBP) per library, per title.