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

1. Historical Auction Data:
- The grade of 80 (MBC+) has had hammer prices that vary significantly. For example, in previous auctions, hammer prices for coins with similar grades have ranged from 170 to 700 euros, depending on demand and other factors.

2. Average Hammer Price:
- The average hammer price for coins of this reference is approximately 406.31 euros. This suggests that, in general, buyers are willing to pay around this amount.

3. Price Trends:
- Observing the trends, it seems that coins with a grade of 80 (MBC+) tend to reach hammer prices that can be slightly higher than the average starting price, especially if there is interest in the auction.

4. Additional Considerations:
- Given that the starting price is 250 euros and the average hammer price is 406.31 euros, a reasonable maximum purchase price could be slightly higher than the average hammer price, considering the grade and demand.

Estimated Maximum Purchase Price:
Taking all these factors into account, an estimated maximum purchase price for the coin with a grade of 80 (MBC+) could be around 450-500 euros. This allows for some competition in the auction and reflects the price trend observed in the past.