To break the values down further I'll explain it like this.
Mana Crystal gives the controller increased channeling, not mana. Channeling by extension gives you mana. So the (time of game-5) does give you it's effectiveness on a simple terms level. However, don't confuse channeling with just getting mana because Channeling has another benefit. That is, it increases the amount of mana you may cast on any given turn. In the long run my opponent may have gained much more mana than I but I was still able to cast larger spells earlier because I'm gaining mana faster, as opposed to slower but consistent.
Here's a turn by turn count.
Lets say 2 mages have 10 channeling.
Turn 0 = 10 mana <------ for the sake of removing values we don't need because the starting value is the same for everyone and it doesn't change the math at all.
Turn 1 = Channels 10 mana.(10) Plays mana crystal (-5). End Turn = 5 mana left. [For the sake of simplicity each turn both mages will spend 5 mana so that I can illustrate the concept of acceleration and not simple mana totals]
Turn 2 = Channels 11 mana.(16) Plays card (-5) End turn = 11 mana left
Turn 3 = Channels 11 mana.(22) Plays card (-5) End Turn = 17 mana left
Turn 4 = Channels 11 mana.(28) Plays card (-5) End turn = 23 mana left
Player 2
Turn 1 = Channels 10 mana.(10) Plays card (-5) End turn = 5 mana left
Turn 2 = Channels 10 mana.(15) Plays card (-5) End turn = 10 mana left
Turn 3 = Channels 10 mana.(20) Plays card (-5) End Turn = 15 mana left
Turn 4 = Channels 10 mana.(25) Plays card (-5) End turn = 20 mana left
Hopefully you see the pattern by now as the ever increasing ratio starts to benefit the owner of mana crystal. This ratio is described by the clause in my equation being (X*1) where x = the amount of rounds that the mana crystal is in play. "1" is a placeholder because these equation could describe the same relationship if multiple were in play.
So in conclusion, not only does the card grant you mana after turn six (which really is just a bonus for the main benefit), you gain a rate of gain bonus over your opponent which allows you to play larger more quickly. Even though you "wasted" 5 mana. Though by my calculations the "value" of mana crystal is zero after 2 and a half turns so each turn after turn 3 is when you start to reap benefits.
Now let me jump the gun here and ask myself, "why is the ratio bonus as equal to the flat mana gain bonus that takes place after 6 turns?" Well, the fact that the ratio bonus is equal is only relative to how much of an advantage it gives me over the other player. So in my opinion the values are equal, but maybe you aren't like me and think the ratio increase is worth only half that of the bonus mana crystal gives you after 6 turns. So plug in ".5" where the "1" is on the equation. The result is that it still only takes 3.33 turns to "pay itself off." Even if you put in ".25" its still a better outcome than the typical 6 turns that people think.
I hope that long winded explanation helps you to understand my point of view a bit. The old simple model isn't really wrong it's just not completely right and downplays the effects of mana crystal and the benefits.