As Greg said, not that much. The costs of physically printing and shipping books are a fairly small percentage of the total cost of making them. People tend to think of the cost of a thing in terms of material expenses, but that's overlooking labor. A lot of people devote a lot of time and work to writing, editing, approving, copyediting, proofreading, and typesetting a book, designing and creating its cover art and text, drawing up and processing the contract for it, writing its catalog entries, marketing and promoting it, etc.
Another thing to take into account is economy of scale. A lot less people would be buying an eBook than a physical book. Using the SCE eBooks as a baseline, eBooks tend to have 1/4th the content of a MMPB, which is sold for $7.99, and 1/6th the content of a TPB, sold for $16 (and in reality ~ $11 if ordered new on Amazon around the time of release and even cheaper if remaindered).
On Amazon, the uncollected Corp of Engineers and TNG Slings and Arrows are sold for $6.99 and Christopher Bennett's Kinshaya eBook can be pre-ordered for $5.99. Let's use the Kinshaya one as a baseline since its profitability will likely determine how soon further eBooks would be published. That makes the proportional content of an eBook cost $1.99 should it be in MMPB terms or $2.67 in TPB terms. Yet the cost to buy one is $5.99. If anything, eBooks are far more expensive per capita than MMPB or TPBs. A TPB at the Kinshaya baseline would cost $35.94 and a MMPB $23.96.