These are all approximate costs, but I think realistic in time.
A new digital book (fiction) should normally cost $9.99.
In 2 years, the cost of the 9.99 book should be no more than 7.99. (This is because there is not a used market.)
Exceptions for the above would be
1. If it is a really greatly anticipated book and then for the first year it should cost as much as the market will bear. Also it might be a very large book. Or a great author.
2. If it is an author trying to get a build up acceptance and their name and willing to cut prices in order to gain a following.
((Remember, no trees are involved in printing these books, no printing presses or binding, very little shipping costs. Yes editing, writing, merchandising, advertising, etc., still go into the book, but the great material costs go way down.))
For non fiction there are other factors so I am not so sure of the pricing.
I could see good non fiction books maintaining their price for years even if sales aren't high. Again, a different situation.
Now after I have said the above I will say this.
Any author, or publisher, or book seller can price their books at any price even if the price exceeds any reasonable logic. At least they can in America.
However if Amazon wants to buy their $19.99 book and sell it for $9.99 it has a right to do that too.
Personally I wouldn't do it, but then I am not as smart and rich as Jeff Bezos.