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

1. Current Condition: The coin has a condition rating of 25 (BC), indicating that it is in a fairly deteriorated state. This generally reduces its value compared to coins in better condition.

2. Historical Prices: By observing the hammer prices of similar coins in the past, we can see that the hammer prices have varied considerably. For example, in previous auctions, hammer prices have ranged from 55 to 301 euros, depending on the condition and other factors.

3. Starting Price and Average Hammer Price: The current starting price is 30 euros, and the average hammer price in the past has been 115.58 euros. However, given that the condition of this coin is low, it is likely that the hammer price will not reach the average.

4. Sales Trends: The coin has been auctioned 23 times, of which 20 have been sold. This indicates a consistent interest in the coin, although the condition may affect demand.

5. Comparison with Similar Coins: In previous auctions, coins with similar conditions have had hammer prices that varied, but are generally lower than those of coins in better condition.

Taking all these factors into account, an estimated maximum purchase price for this coin, considering its condition and historical prices, could be around 50-70 euros. This reflects a balance between interest in the coin and its current state.