Internet marketing for business presentation and business strategy site

I operate an ecommerce site that specializes in business presentation materials (e.g. PowerPoint templates, diagrams, charts) and business strategy documents (e.g. business frameworks, business templates, financial models). All the products are digital.

Currently, I am relying almost entirely on organic search traffic. Through this, my monthly sales have increased fairly steadily month-over-month -- except for the past couple months. In fact, sales this month has been rather dreadful (compared to my expected sales).

I am now looking for more direct online marketing methods - i.e. advertisements on targeted sites. I'm finding most relevant sites are big name sites (e.g. Harvard Business Review, McKinsey Quarterly, Booz & Co's Strategy+Business), which aren't economical for me. These sites seem to cater more to brand advertisers, whereas I am looking to make an immediate return. I figure I need about a $0.30 CPC to turn a profit. Is this rate not realistic anymore?

In the past, I've also tried AdSense, LinkedIn, Facebook, and BusinessWeek's self-service advertising solutions.

My ideal customer base is mid-management to executives at large companies.

Looking for suggestions on how best to market a site like mine. Would appreciate specific suggestions (e.g. advertise on site X).

Thanks!

Dave