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

1. Starting Price: 400 euros. This is the initial price set for the auction.
2. Current Condition: 80 (MBC+). This indicates that the coin is in good condition, which can positively influence its value.
3. Auction History:
- In 2016, the coin was auctioned with a starting price of 1084.97 euros and sold for 2034.32 euros, with a condition of 75.
- In 2023, it was auctioned with a starting price of 1000 euros and an estimated price of 1300 euros, with a condition of 80.

4. Average Starting and Final Prices: The average starting price is 828.32 euros and the average final price is 2034.32 euros. This suggests that, in general, coins of this reference tend to reach significantly higher prices than their starting price.

Given that the condition of the current coin is 80, which is the same as the last sale, and considering that the starting price is significantly lower than previous final prices, we can estimate a maximum purchase price.

Based on the price history and condition, a reasonable maximum purchase price could be in the range of 1200 to 1500 euros, as this aligns with the estimated price of the previous auction and the fact that the coin is in good condition.

Therefore, an estimated maximum purchase price could be 1500 euros.