@Capn-Franky That didn't apply to empty carts though, I found the answer further down your handy link:|
"It seems as if the optimal Powered Rail placing interval to make storage mine carts move is 4 (1 Powered Rail every 4th block). Compared to shorter intervals the reduction in speed is minimal. The maximal possible interval seems to be 9 as the minecart will not reliably reach its destination when using higher intervals."
So... pretty durn expensive.