Ask HN: DRM alternatives for ebooks/audiobooks

I am in the process of developing an ebook/audiobook platform and I think Digital Rights Management is crucial to gain the publishers' trust. I have only considered Readium LCP at the moment, and it looks like the ideal solution. The only problem is I don't know the certificate fees, and this makes my business planning hard at the moment. I have reached out to them, but no response so far. My question is: what other alternatives do I have to implement DRM? Do you know the associated costs?