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

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

2. Auction History: By observing previous hammer prices for coins with similar conditions, we can see that:
- Coins with a condition of 75 have reached hammer prices of up to 650 euros.
- Coins with a condition of 85 have reached hammer prices of up to 900 euros.
- Coins with a condition of 80 have had hammer prices ranging between 240 and 550 euros on several occasions.

3. Starting Price and Average Hammer Price: The current starting price is 300 euros, and the average hammer price for this coin is 288.82 euros. This suggests that the market is willing to pay close to this price.

4. Recent Trends: In the most recent auctions, hammer prices have shown a trend of being higher, especially for coins in good condition.

Based on these factors, an estimated maximum purchase price for this coin could be between 400 and 600 euros, considering that the condition is good and there is a history of hammer prices that supports this estimate.

Therefore, a reasonable maximum purchase price could be approximately 500 euros.