To estimate a maximum purchase price for the coin in question, we will consider several factors:

1. Condition: The coin has a condition rating of 80 (MS+), indicating that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Previous auction prices:
- In 2015, the coin was auctioned with a starting price of 750 euros and a final price of 800 euros.
- In 2015, it was also auctioned with a final price of 975 euros.
- In 2021, it was auctioned with a final price of 1300 euros, with a condition rating of 85.
- In 2022, it was auctioned with a final price of 1250 euros, with a condition rating of 75.

3. Average final price: The average final price for this coin is 1081.25 euros, which provides a good reference for its current value.

4. Current starting price: The current starting price is 1400 euros, which is higher than previous final prices, but may be justified by the condition and demand.

5. Auction trends: Given that there has been consistent interest in this coin (4 out of 5 auctions have resulted in sales), this suggests that there is an active market for it.

Taking all these factors into account, an estimated maximum purchase price could be around 1300-1400 euros, considering that the highest final price in the past was 1300 euros and the current starting price is 1400 euros. However, since the starting price is higher than previous final prices, it may be prudent not to exceed 1300 euros in bidding, unless there is strong competition in the auction.

Therefore, a reasonable maximum purchase price could be 1300 euros.