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

1. Coin Condition: The coin to be auctioned has a condition of 70 (MBC-). This is an important factor, as coins in better condition tend to reach higher prices.

2. Previous Auction Prices: By observing the hammer prices of coins with similar conditions (70 and 75), we can see that:
- In 2018, a coin with a condition of 70 was sold for 220 euros.
- In 2022, another coin with a condition of 75 was sold for 700 euros, but this may be an exceptional case.
- In 2022, a coin with a condition of 70 was sold for 260 euros.

3. Average Hammer Price: The average hammer price of coins auctioned in the past is 268.1 euros, which gives us an idea of the general value of these coins.

4. Starting Price: The starting price of the coin in the current auction is 300 euros, indicating that the seller has high expectations.

5. Sales Trends: The coin has been sold in 10 out of the 11 previous auctions, suggesting that there is consistent interest in this coin.

Based on these factors, an estimated maximum purchase price could be in the range of 250 to 300 euros, considering that the starting price is 300 euros and that the highest hammer price for a similar condition was 260 euros.

Therefore, a reasonable estimated maximum purchase price could be approximately 280 euros. This allows for some competition in the auction, but it is also based on historical prices and the condition of the coin.