It depends on the number of words you are seeking and whether it is intended for Google only and not for human visitors.
You can get articles written for as low as $1 on odesk. But it really depends on what you are going to use them for whether or not you should pay that low. At that low a price you can pretty much expect garbage and would only use it for like spinned content for web 2.0 sites. I actually have found some pretty decent foreign writers from philliphines and India for low as $3 per article. But if you are putting the content on your site I would look for an native English speaking writer for which you can probably find someone for $5 on the low end to up to the $12 range. If you need a technical article that requires specialized training then I would expect to pay quite a bit more. If you go for one of the freelancing article sites that claim to use English speaking writers then expect to pay around $7 each. So it really is all over the place.
All depends on how much and how competitive your keywords. Normal fee for 5 keywords might be from 100 to 500 USD.
Article writing depends a lot on the quality of the content. I don't think you should focus on the word count. A word count should be flexible and depends on how many words are needed to convey the message. Still for a range I would say, between $10-$15 for a decent article which is written by some research and not just re-writing. You can get cheaper ones as well but they won't be quality and will not help you in getting noticed at Google.
$5 is an average charge for article with unique and fresh content for 350 to 400 words. Now it depends on writer, he may decrease charges, if he get bulk writing order.
When it comes to writing, you definitely get what you pay for. Don't expect high-quality content with no grammar mistakes, typos, or anything else for a buck. We would recommend somewhere like Need an Article if you want article services performed. If you want to hire a freelance writer then consider a place like Elance. This will allow you to post your job and the writers will provide proposals. You can read over examples, reviews from other customers, and choose the best deal for you. Good luck~
I believe that it all depends on the niche that you have and what you're expecting plus how much experience the writer has. Expert writers usually ask for between .02 or higher (tho, sometimes they'll go cheaper if they are in need of money). These days, most people get paid at least .01 per word. However, if you have a niche that requires a lot of research and time then it might be as high as .03 per word.
If you can afford $3.5/100 words, you can try to buy from constant content or ghostblogger.net (use the request articles option) If you can only afford $1/100 words, you can try fiverr (find a gig with good review)