To estimate a maximum purchase price for the coin in question, we can consider several factors based on the provided data:

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

2. Auction History:
- The average hammer price for coins with similar condition (80-85) has been approximately 300 euros on several occasions.
- In previous auctions, coins with a condition of 85 have reached hammer prices of up to 400 euros.
- The price trend shows that, despite the starting price being 300 euros, the hammer price can be significantly higher, especially if there is interest in the coin.

3. Starting Price and Average Hammer Price:
- The starting price is 300 euros, and the average hammer price in the past has been 266.62 euros. However, the hammer price can vary depending on demand at the auction.

4. Recent Trends: In the most recent auction, the coin sold for 380 euros, indicating a growing interest in this coin.

5. Comparison with Previous Prices:
- In previous auctions, coins with similar conditions have reached hammer prices around 300-400 euros.

Based on these factors, an estimated maximum purchase price for the coin could be between 350 and 400 euros, considering that the condition is good and there is recent interest in the coin. Therefore, a reasonable maximum purchase price could be 400 euros.