How you price yourself should depend on the amount of time and research you spend writing your pieces. For example, I often charge $1000 per month for weekly blog posts, which includes writing, research, edit, sourcing photos, and posting the blog for the client.
Blogging is not really ad copy, as blogging is more about getting information out and telling a story or providing value, and is longer than an advertisement and should genuinely provide value — which encourages people to stay on your client’s website for longer. A good blog shouldn’t only be selling you something, though a call to action is always good.
Yes, approaching small to midsize companies is a good idea. Just remember, a blog is often not only technical but also conversational.