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

1. Current Condition: The coin has a condition rating of 80 (MS+). This is an important factor, as coins in better condition tend to reach higher prices.

2. Auction History:
- In the past, the coin has been auctioned 5 times, of which it has sold 4 times. This indicates a consistent interest in the coin.
- Previous hammer prices are relevant. For example, in the 2021 auction, the coin was estimated at 20,000 euros and sold for 19,000 euros, indicating that there is a market willing to pay high prices for coins in good condition.
- In the 2022 auction, although the condition was slightly better (85), the hammer price was 11,000 euros, which is also a data point to consider.

3. Starting Price and Average Hammer Price:
- The current starting price is 10,000 euros, and the average hammer price in the past has been 14,187.5 euros. This suggests that the hammer price could be in a similar or higher range than the starting price.

4. Price Trends:
- The trend of hammer prices has been upward, especially for coins in better condition. The last auction in 2024 shows a hammer price of 15,000 euros for a coin with a condition of 95.

Based on these factors, an estimated maximum purchase price for the coin with a condition of 80 could be in the range of 12,000 to 15,000 euros, considering that the interest in the coin is high and that past hammer prices have been favorable.

Therefore, a reasonable maximum purchase price could be 14,000 euros.