The highest quality and cost is https://contently.com/ - $200+ per post.
Alternatively, I've uses http://jobs.problogger.net/ job board to post an opportunity, and then create a simple "test" for the writers. If they read the instructions and the content is great, try em' out .. if not, then pass.
It takes time but you can find great people who will blog for $30-50 per post.
Another way - not fast but high quality - is to approach existing blogs in your vertical with smaller audiences ... ask them to guest post or ghostwrite for you .. they already know the subject matter and blogging + if they have an audience you can incentivize them to promote the content via their social channel (by paying social share bonuses).
Hope that helps.
I have tried a few of these services as well as going out and finding my own sources which tends to be very time consuming. I would recommend checking out BlogMutt (www.blogmutt.com/). Their pricing starts at $89/month for 4 articles. They vet all of their writers and have gone with more of a contest approach so each week you have a selection of articles to choose from.
They work with you on keywords and you have the option to request edits or reject posts. Equally important they seem to treat their writers well and pay them fairly which makes for more consistent quality.
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.
Often the best way is -- as someone else mentioned here -- to approach bloggers who already write on your topic on their own blog. Plenty of experts have deep knowledge of their topic, but don't know how to turn those ideas into an engaging blog post.
We've got a database of about 500+ bloggers we mine whenever we need writers (we manage blogs for small businesses). Happy to put you in touch with someone who would be a good fit if you'd like: http://www.socialexis.com/need-a-freelance-blogger/
First would be to locate experts from LinkedIn and other professional groups. They are really experts in their respective fields. Others can be found on job sites like Odesk. Some have their own websites which you can search by typing "field + experts + location"..