Determining the optimal price for an eBook is somewhat of a mystery. There are a number of competing ideas about how best to find out what that is. For starters, they're almost always priced less than the printed version. That makes sense for several reasons, such as printing cost and shipping which eBooks don't have.
In looking at Amazon, 80% of the price of the printed copy seems to be a common ballpark figure they use for the eBook. Whether or not that's the best price is unclear, but it's what I'm seeing.