Mastery 6+ costs - a discrepancy

It just occurred to me when burning a Shen shard for mastery 6, that was 630 BE I was losing. Whereas when I got mastery 7 for Yi, I stupidly used 3k BE that should've been a mere loss of 90 BE if it had been just a shard. Besides, the 450 BE champs seem to be more common, I would've gotten it at some point if I had been more patient (for reference, I now have 2-3 copies of pretty much all the 450 champs and only 1 of the higher tier ones.

I feel that it is unfair in a sense. Why should it be harder for a Rakan main to get his M7 than a Soraka main, especially if they've both proven themselves by getting the mastery tokens? The costs add an unnecessary hurdle after the S/S+ games that also discriminate against players using expensive champions. If they use their shards, that's 1.2k BE they're essentially burning off while players who play cheaper champs aren't losing much from burning their shards that are worth 90 BE (on top of being more common).

It's almost never worth to pay the flat BE cost, yet shard usage highly favors the mains of low BE champs.