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.