I got bored at 4:00AM and decided to test the value of GP/10 quintessences and their efficiency in comparison to other stats, namely Ability Power and Attack Damage. The first chunk of this is all mathy, but will be summarized under this bit of text.

***NOTE: The end numericals are not factoring in gold acquired via minions. All times are approximate, assuming the max gold bank has already been reached. Seige Cannon minion gold has been excluded. 9~15% error margin with all end numbers.

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======CONCLUSION AND FURTHER OBSERVATIONS======
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2005g more at 30 minutes w/ bonus start+2 gp/10 in comparison to no rune+mastery and GP/10 start. You would need to have, approximately, a 51 minute long game without the theorized GP/10 setup in order to match the amount of gold gained at 30 minutes with the theorized GP/10 setup, thus the proposal is 41% more efficient when executed.

Based off of the current gold-per-stat values as of the start of the S3 Preseason, taking the GP/10 quints will match the value of AD and AP quints at the following times;
6.75AD= 263g~ (14m36s) [@ base level?]
15AP= 316g~ (17m30s)

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BASE RATE
16g/10s
[24/15s]
96/m
960/10m
2880/30m

=============
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W/ bonus
21g/10s
[31.5/15s]
126/m
1260/10m
3780/30m
=============
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Total Possible Gold w/ 2 mods

25g/10s @ 15m15s
[75/30s]
150/m
1,500/10m
4,500/30m (45m15s realtime)
=============
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30g/10s @ 10m45s
[90/30s]
180/m
1,800/10m
5,400/30m (40m45s realtime)
=============
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Non-Bonus start
960+480= 1440g@15m +++ 2250g@15m = 3690g @30m
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Bonus Start
1260+205= 1465g@11m45s ++ 3420g@19m= 4885g @ 30m

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60 minions= 1,050
120 minions= 2,100
[at max bank value]

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900g difference over 30 minutes with gp/10 quints+masteries
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WAVE GOLD

60g/3melee
45g/3range
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105/wave (six mixed)[EXCLUDING CANNONEER]
105g/30s (perfectCS)
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(first wave @ 2:00)
Theoretical FUll Efficency
===============
210+96=306gp/m

>>3,060gp/10m<< (NO ACCELERATION)
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210+126=336gp/m

>>3,360 gp/10m<< (ACCELERATED BY 5g/10s)
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Theoretical Half Efficency
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105+96=210gp/m

>>2,100gp/10m<< (68.6% efficency) [960g difference]
=============
105+126=231gp/m

>>2,310gp/10m<< (68.8% efficency) [1050g difference]
=============
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=============
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Philo+Kage= 1,465g

w/o boosted gp/10= 15m15s

w/ boosted gp/10= 11m45s

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=============


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NET STAT WORTH

+1 ad: 41.5g | 39g | 36.(6)g - >>39.0(5)g<<
+1 ap: 21.75g | 21.5g | 20g - >>21.08(3)g<<

6.75AD= 263g~ (14m36s) @ base level?
15AP= 316g~ (17m30s)

Quints
3/10s
18/m
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CONCLUSION
2005g more at 30 minutes w/ bonus start+2 modifiers in comparison to no-mod start.

9~15% error margin***


Cheers,
Incendiax