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 70 (VF-), indicating that it is in quite good shape. This generally increases its value compared to coins with lower conditions.

2. Auction History:
- It has been auctioned 5 times in the past, of which it has sold 3 times. This suggests that there is moderate interest in the coin.
- The average hammer price in previous auctions is 60.67 euros, which provides a good reference for the current value.

3. Previous Starting and Hammer Prices:
- In the 2020 auction, the starting price was 45 euros and the hammer price was 51 euros.
- In 2018, the starting price was 60 euros, but it did not sell.
- In 2017, the starting price was 90 euros, but it did not sell, indicating that the starting price may be too high in some cases.

4. Current Starting Price: The starting price is 50 euros, which is reasonable given the price history.

Based on this data, an estimated maximum purchase price could be set between the average hammer price (60.67 euros) and a bit more, considering the condition and interest in the coin.

Therefore, a reasonable maximum purchase price could be approximately 70 euros. This allows for a margin for competition in the auction, given that the condition is good and there is a price history that supports this value.