Um...You sure you have those costs right? I would think that it would be more expensive, rather than have a -1 
cost per copy.
Are you accounting for the drain?
They offset the drain by playing creatures.
Example:
Myriad + Horned Frog + Fractal
Cost: 4
+2
+10
+ 12
+ 3 cards
Generate: 7
Actual Cost: 11
+ 10
+ 3 cards ~= 26
Result: 14 horned Frogs + 1 Myriad for the turn
Unit cost: ~= 26/14~=1.86
Horned Frog + Fractal
Cost: 2
+10
+ 12
+ 2 cards
Actual Cost: 14
+ 10
+ 2 cards ~= 29
Result: 7 horned Frogs
Unit cost: ~= 29/7~=4.14
Notice how Myriad halved the unit cost after taking its cost into account. So yes the quanta generation must go. But is that enough?
Myriad + Horned Frog + Fractal
Cost: 4
+2
+10
+ 12
+ 3 cards
Actual Cost: 18
+ 10
+ 3 cards ~= 33
Result: 14 horned Frogs + 1 Myriad for the turn
Unit cost: ~= 33/14~=2.36
Note that a single Myriad still decreases the unit cost by a lot.
Myriad x2 + Horned Frog + Fractal
Cost: 8
+2
+10
+ 8
+ 4 cards
Actual Cost: 18
+ 10
+ 4 cards ~= 35
Result: 20 horned Frogs + 3 Myriads for the turn
Unit cost: ~= 35/20~=1.75
However when we look at multiple Myriads we can see that free triggered mitosis is ridiculously powerful.
