So World of Warcraft is on a huge discount right now - 25 euros for vanilla + all expansions including Legion. Here's a guide how to get a level 100 boost for only 25 euros.
You can have 8 World of Warcraft licenses on one account and a boost that you get with each of the purchases is ACCOUNT WIDE so you can use it on a character from your main WoW license.
Basically go to account management on battle.net and under "your game accounts" go to World of Warcraft "play for free" - here you will create a new game license. Then buy the WoW bundle (vanilla + all expansions, including Legion) for 25 euros either via website or battle.net app and after that you will be able to use a character boost on your main WoW license.
I just did it and it worked - I have a 100 boost on my main license.
I hope this helps a lot of people.