You get what you pay for for the most part. If you want isolated seeds guaranteed to grow true they're generally not going to be cheap, especially for varieties like bhut jolokia. If you've ever cut open a bhut jolokia then you'll understand why people charge $7+ for 10 seeds. The long, assam/CPI strains are not generally great producers and when you cut open pods there are often only 3-4 seeds in a whole pod. Compare that to a jalapeno or larger annum which may have as many as 50-100 seeds per pod and is shorter season, so you can usually get those varieties for a lot cheaper, but the superhots usually cost a little more, understandably. Considering the cost and time of building and maintaining isolation cages and the reduced yield (generally) of plants growing in these cages, or keeping plants at properties miles apart and driving around to take care of them, and it's easy to see the reason for the prices. On the other hand, some seed sellers do not isolate their plants, and may be a little cheaper, and some may not isolate their plants, and still charge as much as the ones who do.
If you can live with a decent possibility of getting some crossed seeds, then ajijoe is a good bet. He doesn't isolate and doesn't make any guarantees that his seeds will be pure, so that's why his prices are so cheap. There's a good chance if you pay more for seeds from some seed sellers that you may still end up with some crosses anyway, so why not? If you get 10 seeds from him and keep 5 plants, it's pretty likely that at least some will grow true (since the seeds you end up with will likely be from multiple pods), and you can usually figure out which if any are crossed, so you can save your own seeds from the ones that grow true. So if you're trying not to spend much money on seeds that's a good option.
Tradewindsfruit is fairly cheap for a lot of varieties as well. I don't know if they isolate but all my seeds from there have grown true and I don't recall anyone having problems with their seeds growing true.