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

1. Current Condition: The coin has a condition rating of 80 (MBC+), indicating that it is in quite good shape. This generally translates to a higher value compared to coins with lower conditions.

2. Auction History:
- In 2015, the coin was auctioned with a starting price of 300 euros and sold for 1500 euros, indicating significant interest and high value at that time.
- In 2021, the same coin was auctioned with an estimated price of 15 euros, but it only sold for 10 euros, suggesting that interest in that particular coin had decreased significantly.

3. Average Hammer Price: The average hammer price for this coin is 543.33 euros, providing a good reference for its current value.

4. Current Starting Price: The starting price is 50 euros, which is relatively low compared to the average hammer price.

Given that the coin's condition is good and considering the auction history, an estimated maximum purchase price could be positioned in a range between the average hammer price and the highest recorded hammer price.

Taking all these factors into account, an estimated maximum purchase price could be approximately 800 to 1000 euros. This is based on the good condition and the interest it has shown in previous auctions, although market fluctuations and current collector interest should also be considered.