To estimate a maximum purchase price for the coin with a grade of 80 (MBC+), we can analyze the historical data from previous auctions of coins with a similar grade.

1. Data from previous auctions:
- The grade of 80 (MBC+) has had hammer prices that vary, but generally, the hammer prices for coins with this grade have been in a range of approximately 300 to 400 euros in recent auctions.
- For example, in previous auctions, the hammer prices for coins with a grade of 80 have been:
- 340 euros
- 320 euros
- 360 euros
- 350 euros

2. Average hammer price:
- The average hammer price for similar coins has been approximately 323.88 euros.

3. Starting price:
- The current starting price is 400 euros, indicating that the seller expects a higher price than the average hammer price.

4. Recent trends:
- In the most recent auctions, hammer prices have shown a trend of increasing, especially for coins in good condition.

5. Estimation:
- Given that the coin's grade is 80 (MBC+) and considering the starting price of 400 euros, a reasonable maximum purchase price could be in the range of 350 to 400 euros, depending on the competition in the auction and the interest from other buyers.

Conclusion: An estimated maximum purchase price for this coin could be approximately 400 euros, as it is the starting price and aligns with previous hammer prices for coins of similar grade. However, if a more conservative approach is desired, a range of 350 to 375 euros might be more prudent.