It depends on so many factors that it's hard to tell. If you're just counting on Adsense income, then, yeah that's probably true. If you're selling affiliate products from the Wordpress blog, then you probably won't need as many hits, just a different type of hits.
Affiliate products do reasonably well with PPC traffic, but you need to do your homework on your niche. You also need to never put affiliate links directly on your blog, you should use redirect scripts to funnel traffic to the affiliate link.