Reasons never pointed out to why M6 and M7 are expensive.
You will have all without spending 1 single BE if you don't disenchant shards from champions you play.
Just like i did in the past i think many of you just forgot that you can actually upgrade your mastery spending 1 shard of your champion in it. Today we can get tons of champions shards and I personally have more than 50. Each champion shard can give you around 100 BE, but if you just don't disenchant it, it will worth you more than 3K BE by using it to upgrade your mastery!
That's it guys! Think this may help you!