In-house is cheapest per article and most expensive to start, an agency is the reverse, and a freelancer sits between the two and fails in a specific way. The right answer depends on one thing: whether the bottleneck is deciding what to write or getting it written.
Yes, but not the kind of blogging most businesses do. Broad explainer posts are finished. Narrow posts that answer one specific buying question are worth more than they were two years ago.
Stop looking at traffic. Traffic is falling across the entire web for reasons that have nothing to do with your writing, and the four numbers that still mean something are inquiries, inquiry quality, branded search, and what buyers tell you when you ask them.
Between about $2,000 and $15,000 USD a month for most small and mid-sized businesses, and $10,000 to $200,000 USD for a fixed-scope project. The range is that wide because you are buying four different things and most sellers will not tell you which one you are getting.