To estimate a maximum purchase price for the coin in question, let's consider the following factors:

1. Starting price: 150 euros.
2. Current condition: 25 (BC), indicating a fairly low condition.
3. Auction history: The coin has been auctioned once before, with a starting price of 150 euros and a hammer price of 190 euros, with a condition of 30 at that time.
4. Average hammer price: 190 euros, which is the average price it has sold for in the past.

Since the current condition is lower than that of the previous auction (25 vs. 30), it is reasonable to expect that the hammer price will be lower than that of the previous auction.

Considering these factors, an estimated maximum purchase price could be around 150-180 euros, given that the previous hammer price was 190 euros, but the lower condition suggests that the price could be lower.

Therefore, a reasonable maximum purchase price could be 170 euros.