You have to jump through a bunch of hoops to get 70% out of of Amazon. In particular, you give up all pricing control -- not only do you have to agree to a ceiling and floor price, but you have to agree not to sell for a lower or higher price elsewhere, and to allow Amazon to change their selling price at will. Which means if AMZN decide your book would make a really cool free promotional item, they can give away some tens of thousands of copies to draw customers in and pay you bupkis. Your book gets to be a loss-leader, and you're the loser.
$17/$50 is roughly the 35% royalty rate. KDP offers both a 35% and a 70% (less delivery at about $0.15/mb in rounded kb increments) royalty rate[1]; however, the 70% royalty rate is only available in about 14 countries and you fall back to the 35% royalty rate elsewise.
I think royalties drop back down once the price hits above a ceiling price. The floor for 70% is $2.99. Not sure at what point it drops back down to 35%.
