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

1. Current Condition: The coin has a condition rating of 80 (MBC+), indicating that it is in fairly good shape, but not the best possible condition.

2. Auction History:
- The average hammer price for this coin in the past has been 397.86 euros.
- In previous auctions, coins with a similar condition (80) have had hammer prices that vary, but hammer prices have been observed between 220 and 480 euros.
- The most recent auction with a condition of 100 reached a hammer price of 1200 euros, indicating that the value can increase significantly with better condition.

3. Starting Price and Estimates:
- The current starting price is 300 euros, which is a reasonable starting point.
- The estimated price for this auction is 500 euros, suggesting that there are expectations for the hammer price to exceed the starting price.

4. Market Trends:
- Since the coin has been sold every time it has been auctioned, this indicates a consistent interest in the coin, which may influence the price.

Based on these factors, an estimated maximum purchase price could be in the range of 400 to 500 euros, considering that the average hammer price has been 397.86 euros and that the estimated price for this auction is 500 euros.

Therefore, a reasonable maximum purchase price could be approximately 480 euros. This allows for a margin for competition in the auction, given the interest in the coin and its condition.