Ha! It's a simple question, with a less-than-simple answer: Somewhere between $25 and $4,500.
The media release (press release/news release) is a powerful and cost-effective marketing and promotions tool, especially for a growing organization. But it's JUST ONE of many important and effective tools. You have to be sure to use it in the right place at the right time.
Read what Bill Byrne of Remedy Public Relations says about it in his article at Muck Rack.