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

1. Current Condition: The coin has a condition of 30 (BC+), indicating that it is not in the best shape. This generally reduces its value compared to coins in better condition.

2. Current Listing Price and Historical Prices: The current listing price is 90 euros, and the average hammer price in previous auctions is 231.65 euros. However, hammer prices vary depending on the condition of the coin.

3. Price Trends: Looking at the hammer prices of coins with similar conditions (around 70-80), the hammer prices have ranged between 100 and 500 euros in previous auctions. However, coins in better condition tend to reach higher prices.

4. Analysis of Previous Sales: The coin has been sold 23 times in the past, indicating consistent interest. The average hammer price of 231.65 euros suggests that, despite the condition, there is a market for this coin.

5. Condition-Based Estimation: Given that the current condition is 30, it is reasonable to adjust the expected hammer price downward. If we consider that coins in BC+ condition typically sell for 30-50% less than those in average condition (70-80), we could estimate a hammer price in the range of 100-150 euros.

Conclusion: Based on these factors, an estimated maximum purchase price for this coin could be between 120 and 150 euros. This reflects a balance between the current condition and historical hammer prices.