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 25 (BC), which is quite low compared to coins that have been auctioned previously, which have conditions ranging from 30 to 80.

2. Previous prices: Previous hammer prices for coins with higher conditions have been significantly greater. For example:
- A coin with a condition of 80 was auctioned for 45 and 40 euros.
- A coin with a condition of 30 was auctioned for 35 euros.

3. Starting price and average prices: The current starting price is 20 euros, and the average hammer price in the past has been 38.57 euros. However, given that the condition of the coin is low, it is likely that the hammer price will not reach the average.

4. Auction trends: Since this coin has a history of successful sales (sold 7 times out of 7 auctioned), this suggests that there is interest in the coin, although the condition may limit the price.

Taking all these factors into account, an estimated maximum purchase price for this coin could be around 25-30 euros. This reflects an adjustment for the low condition, but also considers the historical interest in the coin.