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 not in very good shape. This generally reduces its value compared to coins in better condition.

2. Auction History:
- The coin has been auctioned 7 times, of which it has sold 6 times. This indicates good demand.
- Previous hammer prices varied, but there is a hammer price of 15 euros in 2019 and 30 euros in 2020 and 2023, suggesting that the price may fluctuate depending on the condition and demand at the time of the auction.

3. Starting Price and Average Hammer Price:
- The current starting price is 30 euros, which is higher than the average starting price of 23.63 euros. This may indicate that the auctioneer expects greater interest this time.
- The average hammer price is 28.33 euros, suggesting that, on average, similar coins have sold for a price close to this.

4. Comparison with Previous Prices:
- In previous auctions, the hammer price has ranged from 15 to 45 euros, depending on the condition of the coin.

Given that the current condition of the coin is 25 (BC), which is relatively low, and considering the price history, an estimated maximum purchase price could be around 25-30 euros. This is based on the fact that, although the starting price is 30 euros, the condition and price history suggest that achieving a higher hammer price may be difficult.

Therefore, a reasonable estimated maximum purchase price would be 25 euros.