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

1. Current Condition: The coin has a condition of 30 (BC+). This indicates that its state is not very high, which generally negatively affects its value.

2. Previous Auction Prices: By observing the hammer prices of coins with similar conditions (30-40), we can see that the hammer prices have varied, but generally, coins in similar conditions have reached hammer prices ranging from 30 to 70 euros.

3. Average Hammer Price: The average hammer price for this coin is 73.87 euros, but this includes coins in better conditions. Given that the condition of this coin is lower, it is reasonable to adjust this value downward.

4. Starting Price: The starting price is 50 euros, which can be a good indicator of what is expected to be obtained at the auction.

5. Sales Trends: The coin has been sold in 89.58% of the auctions it has participated in, suggesting relatively high demand.

Based on these factors, an estimated maximum purchase price for this coin could be in the range of 40 to 60 euros, with 60 euros being a reasonable upper limit considering the condition and price history.

Therefore, an estimated maximum purchase price could be 60 euros.