In the past I've gone to Carol Tice at MakeaLivingWriting.com and asked to posts a job opportunity in her community. She has a large community of writers she works with, and lots of them are quite good.
As others have mentioned, finding someone who understands your topic is a great place to start. Freelancers will write for less, but they'll need to learn your subject matter which takes time. If you can find someone who's already an expert, then there's a greater likelihood you'll get higher quality articles.
So how much should you pay and how do you find experts? One way is to reach out to people who write guests posts on blogs that you read. If you like someone's style and expertise, you can reach out to see if they'll write for your site. You'll need to pay a minimum of $100 and possibly more. You can also sell it as the base pay plus added exposure for writing for your site. There are several experts who will work on this kind of arrangement, although the top tier will require more than $100 to $150 per post.
Another option, as Dan Martell mentioned, is paying a base price plus a sharing bonus. Let's say you pay $125 per post + a bonus based on how much it gets shared, then the writer is more likely to share the post once it's live. If you aren't paying a bonus, then there's no reason to share. Let's say the bonus ends up being $50 to $75, then you just paid a small fee to get more visitors and shares for a post you already paid good money to have written. Lots of people overlook promotion when it comes to blogging, but it's just as important as awesome content.