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

1. Condition: The coin has a condition rating of 75 (MBC), indicating that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Auction history:
- The average hammer price for this coin has been 668.63 euros, suggesting that, in general, buyers are willing to pay significantly higher prices than the starting price.
- In the past, hammer prices have varied considerably, ranging from 40 euros to 4600 euros, depending on the condition and other factors.

3. Starting price and previous prices:
- The current starting price is 40 euros, which is relatively low compared to the average hammer price.
- In previous auctions, the highest hammer price was 4600 euros for a coin with a condition of 95, indicating that the value can increase significantly with good condition.

4. Sales trends: The coin has been sold in 8 out of the 9 times it has been auctioned, indicating a consistent interest in the coin.

Given all this, an estimated maximum purchase price could be set in a range that reflects both the current condition and the price history. Considering that the condition is 75 and the average hammer price is 668.63 euros, a reasonable maximum purchase price could be in the range of 600 to 800 euros.

Therefore, an estimated maximum purchase price could be 800 euros.