To estimate a maximum purchase price for the coin in question, let's consider several factors:

1. Current starting price: 1200 euros.
2. Coin condition: 100 (S/C), indicating good condition.
3. Auction history:
- In 2020, the coin was auctioned with a starting price of 1500 euros and sold for 2600 euros, with a condition of 110.
- In 2021, it was estimated at a price of 2000 euros and sold for 1500 euros, with a condition of 100.

4. Average starting and final prices:
- Average starting price: 1350 euros.
- Average final price: 2050 euros.

Given that the condition of the current coin is 100, similar to the 2021 auction, and considering that the final price on that occasion was 1500 euros, we can infer that the final price could be in a similar range.

Additionally, the current starting price is lower than the average starting price and the final price of previous auctions, suggesting there is potential for an increase in the final price.

Taking all these factors into account, an estimated maximum purchase price could be around 2000 euros, which is a reasonable value considering the auction history and the condition of the coin.