Rather than blurt out the basis of the story in what is essentially a public space,
The fact that this is a public space shouldn't deter you from going into detail. One of the hardest things to accept about writing, especially to non-writers, is that the skill of writing -- whether technical, trained, or natural -- is far more important than the idea behind the novel.
An idea without a writer is just a thought, never going anywhere, but a good writer can always write, even if they use a generic or borrowed idea. Even if your idea truly is unique, or at least some of the details of it are rarely expressed, three different authors basing stories on that idea will create at least three different finished works, more if one decides to play with variations on a theme. In some cases, the finished stories might be so different as to make the originating idea invisible except to those who know to look for it.
Another thing to think about is the difference between an idea and an outline. Saying, for example, that you want a story based on an alien invasion where it turns out later in the story that the aliens are actually time-travelling humans, genetically engineered for different biomes, is an idea. From that idea you could end up with any number of interpretations.
From your description, though, it sounds like you have a very specific structure in mind for the finished product. Now that's fine, but it does require that you do a lot of the work. You can't just give a hired author a general sketch; the more specific you want the finished project to be, the more detail and organisation that you need to put into your outline.
As for remuneration, a lot will depend on the situation. Most of the authors on a site like SOL are writing for fun; while some do also sell their work, writing isn't generally their main source of income. That is both good and bad for you. It is good, because it means that writing your story doesn't take them away from a five-figure project, but bad because it means that many available writers will only take on projects that they are also enthused by.
That brings us back to your going into detail on the forum. If you just talk to a couple of specific authors, that's a work for hire situation and could cost some serious money, but if you sketch out your idea more fully, you might attract the attention of someone who actively wants to write that story, and will be willing to do so for far less money.