I suggest you try something new rather than providing isolated, paywall-protected pieces of material. Upgrades to short-form content that go into more depth on the topic covered can be provided as a call to action at the piece’s conclusion. This will capitalise on people’s FOMO (fear of missing out) while providing a useful service. A conceptual post about PPC followed by a content upgrade of a PDF with step-by-step instructions for Google PPC newbies is one example. Slideshare presentations, podcasts, checklists, etc., are all possible kinds of content enhancement.