It's honestly down to preference but the way I think of it is like this.

The gold efficiency of +20 AP is = 435 gold Normally I leave lane phase with 35-50 AP so about 800 gold worth of ap stacked up.

Targons value is 865. Sale value of targons is 350. If we make minions average value 20 gold and I proc targons 30-40 times thats 600-800 gold. So off targons alone you profit when it is sold by 100-300 gold and it reduces the need for investment in some hp pots. Not to mention it allows you 800-1000 gold worth of AP.

Now we aren't in lane phase and I'm still last hitting with targons and getting stacks on waves that come crashing into us until I can afford to sell it for talisman. Say this would go on until 22-25 minutes and ill collect an extra 25-30 AP with prefect usage.

Total value in the range of 1700-2000 in 22-25 minutes. The AP value cannot be gotten without going targons early, but going straight coin could allow for more opportunities to get the team kills.

Now lets discuss coin. Say we hold it for 5 minutes and back at 7 minute then upgrade. In that time assuming we are near all minion deaths. Thats approximately 63 minions that die so 199 gold and some mana regen that we dont really need.

Then we upgrade to nomads and hold that until say 18 minutes for the sake of making this close. We won't be near every creep death but lets assume we are near around 85% of them to give this a chance. 11 minutes with 2 gold per 10 is 122 gold. Thats 119 creeps with 4 gold per each one. Bringing the total gold earned from going directly towards talisman up to 797 gold.

Now we hold 797 gold and nomads which in total has a value of 1662. You don't have much extra ap except for the last hits of stray cs and assist AP.