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

1. Starting Price: 100 euros. This is the initial price expected to be obtained at the auction.
2. Coin Condition: 75 (MBC). This indicates that the coin is in fairly good condition, which may positively influence its value.
3. Auction History:
- It has been auctioned 3 times in the past, of which it has only sold once.
- The average starting price in previous auctions was 70.43 euros, and the average hammer price was 40.86 euros.
- In the 2015 auction, the hammer price was 40.86 euros, and in 2021, the estimated price was 58.21 euros.

Given that the coin is in good condition and the starting price is 100 euros, it is reasonable to assume that interest in the coin could be higher in this auction. However, the sales history shows that it has not always sold for high prices.

Taking into account the starting price and the history of hammer prices, an estimated maximum purchase price could be positioned between the starting price and the average hammer price of previous auctions.

Therefore, an estimated maximum purchase price could be around 80-90 euros. This reflects interest in the coin but also considers the history of lower prices in previous auctions.