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

1. Current starting price: 1200 euros.
2. Auction history:
- In 2020, the coin was auctioned with a starting price of 700 euros and sold for 1200 euros, with a condition rating of 80.
- In 2015, the starting price was 900 euros, but it sold for 950 euros, with no information on the condition.

3. Average starting price: 933.33 euros.
4. Average final price: 1416.67 euros.

Since there is no information on the condition of the current coin, it is difficult to make an accurate estimate. However, we can consider the following points:

- The highest final price in the past was 1200 euros, which matches the final price from the 2020 auction.
- The average final price (1416.67 euros) suggests that, in general, coins of this reference tend to sell for above their starting price.

Taking these factors into account, an estimated maximum purchase price could be in a range between the average final price and the highest final price observed.

Therefore, an estimated maximum purchase price could be approximately 1400 euros. This is based on the trend of previous final prices and the fact that the current starting price is 1200 euros.