To estimate a maximum purchase price for the coin in question, we can analyze the provided data:

1. Current starting price: 1100 euros.
2. Coin condition: 95 (S/C-), indicating good condition.
3. Previous auction prices:
- In 2020, it was auctioned for 1500 euros with a starting price of 800 euros.
- In 2018, it was auctioned for 1460 euros with a starting price of 1200 euros.
- In 2015, it was auctioned for 1425 euros with a starting price of 800 euros.
- In 2021, it was auctioned for 1100 euros and 1200 euros in different auctions with a condition of 95.

4. Average auction price: 1379.38 euros, indicating that, in general, coins of this reference tend to sell above the starting price.

Given that the coin is in good condition and considering the previous auction prices, an estimated maximum purchase price could be set in a range between the average auction price and a bit more, since the starting price is 1100 euros.

Estimated maximum purchase price: 1500 euros.

This price is based on the historical behavior of the coin in previous auctions and its good condition.