One must be wary of faux precision. But I think I would put the odds of >100% or <-40% at under 30%; I'd assign another 10 or 20% to a gain between 30% and 100%, and leave the rest to the range of small losses/gains.
The ten categories I suggested may be a bit excessive, but it would be much easier to judge if you were a little more precise. You acknowledge a non-trivial chance of losing a non-trivial amount of money. The confusion is that I thought your previous statement that a "smart bias-educated person can beat the prediction markets fairly easily" would preclude this.
Here's our place to discuss Less Wrong topics that have not appeared in recent posts. Have fun building smaller brains inside of your brains (or not, as you please).