Entendo mais do que imagina, mas como aqui é um site internacional bora responder em inglês.
They didn't expressed very well but their math is right:
The 21 frags for a unique mega will cost on total 490 frags for the first time, 70 from 10 frags exchange and the rest from 30 frags.
But if you want to get more 21 pieces to use on another mon to get the same same mega, you'll need 630 frags since you'll not have the 10 frags per piece option anymore.
That of course counting with frags alone, you can get a piece in random drops. I got 3 from Phoenixmon already farming frags at its Expert stage (easiet one for my mons to farm right now).