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

1. Current Condition: The coin is in a condition of 90 (EBC), which indicates that it is in good shape. This generally increases its value compared to coins in lower conditions.

2. Auction History:
- The average hammer price for similar coins in the past has been 43.53 euros.
- In previous auctions, coins with a condition of 90 have reached hammer prices of up to 120 euros (as seen in the auction on 2021-10-20).

3. Starting Price and Estimates:
- The current starting price is 30 euros, which is reasonable given the history of starting and hammer prices.
- The estimated price for this auction is 70 euros, suggesting that bidders expect the coin to reach a higher price.

4. Sales Trends: Out of the 26 times it has been auctioned, 20 have resulted in sales, indicating good demand.

Based on these factors, an estimated maximum purchase price could be between 80 and 100 euros, considering that the condition is good and there is a history of hammer prices that supports this estimate. However, since the starting price is 30 euros, it is likely that bidders could acquire it for a price below this range, depending on the competition in the auction.

Therefore, a reasonable maximum purchase price could be 90 euros.