First, read this page about the Royal Rank.
http://wotvffbe.gamea.co/c/1e1g0t3h

Reaching Roryal Rank 6 will cost about 1500 US$. I however recommend you to carefully calculate by yourself before you spend money for that.

WRONG. Buy 1 79.99, 1 49.99, 3 23.99, and a couple 7.99.. how do i know? I am Royal Rank 6...IGN: Legits

I'm sorry if I made something wrong. I however don't get what you are mentioning?

he’s saying you’re fos for saying RR6 in $1500. Spoiler alert, you get a higher RR for $1500

I read the question again and now I found that it's asking how much does it cost to reach Rank 6 for "Two exploration slot". I was thinking it's asking how much to unlock "2x Speed function". I'm sorry.