The way I understand it, with Rail Passes, you just buy the pass, and when selecting whatever trains/segments you'll be taking, choose to "add a sleeper upgrade" for the segment. Amtrak sleeper pricing is the base "rail fare" for each room occupant, plus the cost of the room itself (or the "accommodation charge"). The rail fare is constant for any date of travel, as it is just the lowest possible Value coach fare, while the accommodation charge will fluctuate between a set of five fare buckets. Since with a Rail Pass booking, your rail fare has effectively already been paid for, you just pay the accommodation charge, and that's it.
Now, the main factors in sleeper pricing for a given itinerary are the remaining availability for that accommodation, the amount of time left until the date of travel, and in some cases, the time of year. There are cases where a room may immediately go to the cheapest bucket when first put on sale 11 months from departure, however many members (including me) have found that in many more cases, it will actually start at one of the higher buckets, and later drops to a lower or lowest bucket about
five months out. There are definitely exceptions to this, and if there is a considered "off-season", the prices will likely drop for that period to reflect it. And finally, but probably most importantly, as a room type sells, the price goes up to reflect supply and demand (this is one factor that really doesn't have any exceptions). To get a sense of what the possible buckets for your itinerary are, and therefore what fare you're looking for, use
Amsnag, a tool that lets you see pricing over up to 30 days at a time. Good luck!