I calculate with an upgrade cost of 4000 coins per unit.
*-upgrade girls give 1080 girl exp
**-upgrade girls give 2700 girl exp
***-upgrade girls give 7200 girl exp

80 stamina bonus stage on average give
0.66 ***-upgrade girl (7200 girl exp each)
2.66 **-upgrade girl (2700 girl exp each)
Total 12000 girl exp.
Girl exp per stamina 12000/80 = 150.
Exp 134
Exp per stamina 143/12 = 11.92

12 stamina bonus stage on average give
1.5 **-upgrade girl (2700 girl exp each)
0.5 **-upgrade girl (1080 girl exp each)
Total 4590 girl exp.
Girl exp per stamina 4590 / 12 = 382.5
Exp 160
Exp per stamina 160/80 = 2

In my calculation I also use all normal seed
2,4% give *-upgrade (1080 girl exp and upgrade cost 4000 coins)
92,0% **-girl (sell for 250 coins)
5,3% give ***-girl (270 girl exp and upgrade cost 4000 coins)
0,3% give ****-girl (360 girl exp and upgrade cost 4000 coins)
Based on this I get that 1 normal seed give -0.9 coin and 0.4131 girl exp.

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Stamina for 10x 2-1 + 4x bonus stage 12
10x 15 + 4x 80 = 670 stamina.

My calculation of coins net from 10x 12-3 and 4x bonus stager (80 stamina)
10x 3171 =31710 coins (10 time stage 12-3)
4x 8032 = 32128 coins (4 time bonus stage)
48x 250 = 12000 coins (selling 48 **-girls from 12-3)
8400 *-0.9 = -7500 coins (8400 seeds = 10x 564 + 4x 690 seeds) upgrade cost minus sell for the **-girls.
-4x 3.33333 * 4000 = -53333 coins (upgrade cost of the 4x 2.66666 **-upgrade + 4x 0.666666 ***-upgrade from bonus stage.
Total 14946 coins. (Same result as shown at before)

My calculation of girl exp (stage 12-3 incl bonus stage)
Girl exp 8400 (seed) * 0.4131 + 4x 12000 = 51470 girl exp (Same result as shown at before)

Girl exp per stamina 51470/ 670 = 76.8

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Stamina for 10x 2-1 + 4x bonus stage 12
10x 12 + 4x 12 = 168 stamina.

My calculation of coins net from 10x 2-1 and 4x bonus stager (12 stamina)
10x 880 =+8800 coins (10 time stage 2-1)
4x 1500 = +6000 coins (4 time bonus stage 12)
28x 250 = +12000 coins (selling 28 **-girls from 2-1)
800 *-0.9 = -720 coins (8400 seeds = 10x 564 + 4x 690 seeds) upgrade cost minus sell for the **-girls.
-4x 2 * 4000 = -32000 coins (upgrade cost of the 4x 1.5 **-upgrade + 4x 0.5 *-upgrade from bonus stage).
Total -16480 coins. (Same result as shown at before)

My calculation of girl exp (stage 12-3 incl bonus stage)
Girl exp 800 (seed) * 0.4131 + 4x 4590 = 18690 girl exp (Same result as shown at before)

Girl exp per stamina 18690/ 168 = 111.25

If I sell ***- girls instead of using them for upgrade coin earned and used will balance for stage 2-1 bonus stage.
Almost all my ***-girls have been fully unlocked already.

Originally Posted by Opalia24

I calculate with an upgrade cost of 4000 coins per unit.

This right here is a big problem with your calculations. The coin cost per fodder used is the same as the knight's level times 100. This means that once you hit level 40, then sure your upgrade cost will be at 4000 coins per fodder, but once you're above that it will be a lot more. The exp required is a LOT more too. It took me about 6-7 level 100 fodders to get a girl from 50 to 60 and at that point the cost is 5k coins per fodder.

Imagine trying to do that with level 5 and level 20 fodders, which do not even grant half of what level 100 fodders give. Heck, imagine trying to level a 5* knight to 70 with just level 20 fodders. You'll get about 5 levels at most each time you feed a full 10 of them above 50 and each time is going to cost you 50k+ coins. Meanwhile 10x level 100 fodders will cost about 50k and get you way higher.

Sure, if you only have bronzies lowbie fodders might still be a thing, but once you start getting a fair deal of gold units (either through gacha or events) the costs will pile up somewhat insanely much.