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 85 (ebc-), indicating that it is in quite good shape. This can positively influence its value.

2. Auction History:
- In the past, it has been auctioned 4 times and sold on all occasions, suggesting that there is demand for this coin.
- Previous hammer prices have varied, but the highest was 140 euros for a coin with the same condition (85) in October 2021.

3. Starting Price and Average Prices:
- The current starting price is 90 euros, which is higher than the average starting price of 61.25 euros, indicating that the seller has high expectations.
- The average hammer price is 87 euros, suggesting that, in general, coins of this reference sell close to this price.

4. Price Trends:
- The estimated price of 150 euros in the October 2021 auction was not reached, but it indicates that there is potential for a higher price in the future.

Based on these factors, an estimated maximum purchase price could be between 120 and 140 euros. This is because, although the starting price is 90 euros, demand and price history suggest that buyers are willing to pay more for coins in good condition.

Therefore, a reasonable maximum purchase price could be 130 euros.