But are they able to write $100 articles? No, they aren't! People in third world countries has a bad education. Your education (probably in UK) did cost the state a lot, this is why you are where you are now. If someone study in MIT and his studies cost him $300K; he won't probably go for a $30K/year job, but will ask for at least $100K. If someone in my country study in one of those free universities (supported by the state), he'll probably spend as low as $2K/year and will be happy with a $5K/year job.
Depending on how 'merely theoretical' your question was, Allow me to provide an example from an entirely different field, where I am not the provider but the buyer. I do a lot of research whenever I purchase a product and a service, comparing specifications and reading reviews (where possible), downloading pictures or reading comments (when there are no objective criteria), and so on, and so on. When I think I found the right person for a particular job, I send them examples of what does not live up to my expectations and examples of what I liked and what I want. This eliminates a large percentage of possible misunderstandings later, because the potential candidates have a means of "orientation". If their samples are not good enough, I would ask them for recommendations. When I am happy or if the work was good but simply not my style, I pay them for the sample, too. The better you did your homework and the clearer defined your objectives, the quicker and less hassle the entire process of service procurement. Obviously, the best doesn't come cheap ... Moreover, this is for graphics designers, 3D prototypers, sculptors and concept artists, not writers. Nevertheless, providing examples of what you like might be better than merely basing your decision on a couple forum posts. One last thing: if you are looking for real content and not just sh*** that attracts web spiders, don't forget about the research. Somebody who goes the extra mile to investigate details, explain them in a comprehensive way, etc., may be a better 'writer' than somebody who is a funny author and has a way with words but no intention to make an effort or no clue what he's talking about. The first guy might get the wording wrong or be the enemy of orthography, but you can easily have that corrected by your secretary. Especially when it comes to technology or other types of advice, your visitors will value the quality of the contents higher than some funny anecdotes, as eloquently written as they may be.
The top writers don't need to advertise. They work primarily on referrals and through other networks. Active writers are often contacted through PM on sites like DP and others without seeking work simply because they have a reputation and established brand. I've seen a couple of writers I know are excellent post in the thread already, and I'd wager neither has a thread anywhere near the BST section.