I only own advanced coils, so I can’t confirm how much per 100 power, only per 300 power.
I have a table (percentage in relation to the 0 coil cost) for the refined titanium mass craft example:
| Adv. Coils | % of Total | Time (seconds) | Spark |
|---|---|---|---|
| 0 | 100.00% | 3000 | 20000 |
| 1 | 76.62% | 2298 | 15323 |
| 2 | 69.69% | 2090 | 13938 |
| 3 | 64.05% | 1921 | 12809 |
| 4 | 59.51% | 1785 | 11902 |
| 5 | 55.94% | 1678 | 11187 |
| 6 | 53.17% | 1595 | 10633 |
| 7 | 51.08% | 1532 | 10216 |
| 8 | 49.55% | 1486 | 9909 |
| 9 | 48.47% | 1453 | 9693 |
| 10 | 47.73% | 1431 | 9546 |
| 11 | 47.26% | 1417 | 9452 |
| 12 | 46.99% | 1409 | 9397 |
| 13 | 46.84% | 1405 | 9368 |
| 14 | 46.78% | 1403 | 9355 |
| 15 | 46.76% | 1402 | 9351 |
| 16 | 46.75% | 1402 | 9350 |
| 17 | 46.75% | 1402 | 9350 |
| 18 | 46.75% | 1402 | 9350 |
| 24 | 46.75% | 1402 | 9350 |
Edit: As you can see, after you hit the 50% mark the diminishing returns kick in pretty heavily. Still worth to go up to 12-14, particularly for recipes with large spark costs / long crafting times (like shop stands for example).