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

1. Current Condition: The coin has a condition rating of 80 (MBC+), indicating that it is in quite good shape. This is important because coins in better condition tend to fetch higher prices.

2. Auction History:
- The average hammer price for previous auctions is 692.88 euros.
- The last auction with a similar condition (80) had a hammer price of 481 euros.
- In other auctions, hammer prices have varied, but they have generally been above the starting price.

3. Price Trends:
- The current starting price is 400 euros, which is slightly lower than the average starting price of 397 euros in previous auctions.
- However, the estimated hammer price for this auction is 650 euros, suggesting that there are expectations for the coin to sell at a higher price.

4. Comparison with Previous Prices:
- In previous auctions, coins with a condition rating of 80 have reached hammer prices ranging from 481 to 550 euros.
- The most recent auction with a condition of 10 is not relevant for this estimation, as the condition is significantly lower.

Based on these factors, an estimated maximum purchase price could be in the range of 600 to 750 euros, considering that the condition is good and that the price history suggests that coins in similar conditions have reached higher prices in the past.

Therefore, a reasonable maximum purchase price could be 750 euros.