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 (MS-80). This is an important factor, as coins in better condition tend to reach higher prices.

2. Auction History:
- The average hammer price for similar coins has been 300.43 euros.
- In previous auctions, coins with a condition rating of 80 have reached hammer prices of up to 320 euros (as seen in the auction on October 21, 2020).
- There are also examples of coins with higher conditions (such as 95 and 100) that have achieved higher hammer prices, indicating that condition significantly influences price.

3. Starting Price and Average Starting Price:
- The current starting price is 275 euros, which is slightly higher than the average starting price of 240 euros. This may indicate an expectation of greater interest in this auction.

4. Recent Trends:
- In the most recent auctions, hammer prices have ranged from 300 to 445 euros, suggesting there is growing interest in coins of this reference.

Based on these factors, an estimated maximum purchase price for this coin could be in the range of 320 to 350 euros. This is based on the historical hammer prices for coins with a similar condition and the current interest in the auction.

Therefore, a reasonable maximum purchase price could be 350 euros.