the big issue is that spellthief lines have the highest "potential" gold and add damage to your attacks, but it is hard to utilize that potential except with point and click mechanics and the stats it contributes are worthless by mid game
(i've actually taken a liking to it on adc's and etc instead of doran's blade because the item is relatively ineffecient. Spellthief on the other hand (since it's worthless to upgrade for 500 gold) gives roughly the same amount of damage, mana regen(a bonus), and 5 gold for each time you hit an enemy (even more if you go utility enhancements), meaning a significant gold advantage at only the cost of early game minor sustain/health, which is typically mitigated by the support line of enhancements
having an extra hundred gold + what you gather really helps until you hit very late game, for instance, tristana/caitlynn can hit from maximum poke distance, if you could utilize everything at the "highest" possible efficiency, then you generate 15(spell thief) + 18(mastery) + 6 (passive generation of item) = about 39 gold per 30 seconds
although the optimal situation is never possible, you can assume that that will give you that much gold extra per minute if you are able to keep 50% efficiency, or essentially a free kill's worth of gold in the first 10 minutes
comparing potential gold values:
spellthief(basic since it's worthless to upgrade): (if combined with the on hit mastery) about 1.3 gold per second
the other line, the coin, is more reliable and allows for passive collection and healing, provide somewhat decent sustain by mid game(roughly usable endgame) gold (combined with passive for fairness): 1.2 gold per second
then face of the mountain for better heals and decent farming (not overpowered at any stage of the game, nor underpowered)
this item is the least gold efficient because it has to upgrade to utilize its maximum gold potential as at targon's brace, it recharges 1 charge per 30 seconds, with both hits a minute, this comes to 32 - 60 gold (based upon cannon minion with a normal minion and of course 2 normal minions), if combined with masteries, it would come to: 1.13 gold per second to 1.6 gold per second
but these are optimal scenarios and unlikely to be achieved
spell thief will grant constant gold early game and some damage and does not require to be upgraded while also being cheap
the coin line will provide constant money so long as you are in range of minion deaths ( a safer bet for most supports)
and the relic shield line will provide decent gold early game while continuing to be fairly stat efficient mid and late, though still better off traded in
OVERALL, THE OPTIMAL GOLD EARNINGS ARE DETERMINED BY PLAYSTYLE AND CHOICES