Would you believe , Zero ! But be careful we writers are little fish in an ocean full of Sharks.
Take a look at www.Lulu.com they handle both hard copy PoD books and allow you to set up E Books as well and the cost is zero. Of course you do all the work in setting it up but there is a lot of free help and guidelines.
Where's the catch ? Actually there isn't one, Lulu gets its reward on your success by taking a commission on sales but the marketing effort is down to you.