Levels 1-5 were 100% participation: get a threshold of x amount of mastery points to level up.
Levels 6-7 are now part participation, part performance. The performance part, getting the S rate, is clear to you, however there is also another requirement to level into that mastery, the participation.
Instead of using a threshold of X amount of mastery points, like in levels 1-5, riot has come up with a price for champion shards..
that are the IP equivalent of the levels 6-7 mastery point thresholds that are expected of players to achieve their mastery 6 and 7 achievements.
For example:
(keep in mind these aren't actually the numbers riot uses but it's the process and math behind this example that riot uses)
If it takes 6,000 IP to reach 60,000 Mastery points (the theoretical level 6 threshold) and it takes about 3 champion shards to reach 500 blue essence, then the cost of each champion shard will be 2,000 IP.
(this is assuming that the ratio between IP to mastery points is 1 to 10)
(cost of champion shard in IP) x (champion shards needed to reach blue essence) = (IP required for mastery threshold)
A = (Cost of champion shard in IP)
B = (Champion shards needed to reach blue essence)
C = (IP required for mastery threshold)
so... it's a simple equation of....
A x B = C
This is called Price, or the cost of something.
A:::::::: will always be fixed, it's going to be a price in the store.
B:::::::: will be determined by the blue essence requirement for each level, which is calculated by a fixed number: average blue essence worth of a champion shard.
C:::::::: will be determined by the mastery point threshold requirement for each level, which is calculated by a fixed number or ratio (the ratio between IP and mastery points) (1 to 10 for the purpose of this).
Level 6 Price, or the cost equivalent of level 6:
A = Unknown.
B = 3, (1 champion shard is about 170 blue essence)
C = 6,000 IP (we don't know the threshold but we're going to say 60,000 mastery points) (60,000/10 = 6,000)
This is how we know the price of A is 2,000,
(from these assumed ratios of 1 to 10, and the mastery threshold of 60,000)
A x 3 = 6,000
A = 6,000/3
A = 2,000 IP
Notice how the ratios of 1 to 10 AND the threshold mastery of 60,000 are all connected and related to a calculated value of A? The IP cost of a champion shard?
Level 7 Price, or the cost equivalent of level 7:
2,000 IP X 3.5 champion shards = Unknown
A = 2,000 - fixed price of IP in the store.
B = 3.5 - 600 blue essence is needed, not 500, so you'll need about 3.5 champion shards instead of 3. (600/170)
C = level 7 mastery point threshold IN IP, Since we used 60,000 for level 6 as a starting point, lets calculate the intended level 7, using the price of champion shards (A) and the new requirement of blue essence, or champion shards needed to achieve that blue essence (B)
2,000 x 3.5 = 7,000
C = 7,000... IP required for new threshold, which is 70,000 mastery points.
So it's expected of you to play 6,000 IP worth of games to reach that 60,000 mastery point equivalent to get to level 6
And its expected of you to play 7,000 IP worth of games to reach that 70,000 mastery point equivalent to get to level 7
You can do this with any champ, or the champion you're trying to reach level 6/7 with. It doesn't matter because the performance part of the system IS tied 100% to your champion.
Since the price of the shards will be the same for all levels, to increase the mastery point requirement between 6 and 7, riot increases the necessary blue essence from 500 to 600. (remember that you buy the shards to break them down into the essence to upgrade your mastery level).
Never once are you expected to pay.
It appears that your point of view implies that there is a need for "chance" in order to get the new masteries when in fact it's 100% work; between playing a calculated, expected, amount of games with your champ, and achieving the required S ratings.
It also appears that you're under the false impression that you're forced to pay to achieve these things also, when in fact, you are not.
These two things are ultimately leading to your frustration, but keep in mind that they are misunderstood confusion.