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

1. Starting Price: The starting price is 75 euros, indicating the initial value expected to be obtained at the auction.

2. Auction History: The coin has been auctioned 3 times in the past and has sold on all occasions. This suggests there is consistent interest in the coin.

3. Previous Prices:
- In 2018, the starting price was 30 euros, with a final bid of 45 euros.
- In 2016, the starting price was 40 euros, with a final bid of 40 euros.
- The average final bid in the past has been 42.5 euros (average of 45 and 40).

4. Average Starting and Final Prices: The average starting price of 48.33 euros and the average final bid of 55 euros indicate that, in general, coins of this reference tend to sell for above their starting price.

5. Condition: There is no information about the condition of this coin, which may make its value uncertain. However, given its history of successful sales, this may balance the lack of information about its state.

Given all this, an estimated maximum purchase price could be set in a range above the previous average final bid, while also considering the current starting price.

Taking into account the starting price of 75 euros and the history of final bids, a reasonable maximum purchase price could be approximately 70-80 euros. This is based on the trend of previous prices and the demonstrated interest in past auctions.