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

1. Starting price: 25 euros.
2. Current condition: 30 (BC+), indicating a low condition.
3. Auction history: The coin has been auctioned 3 times and has sold on all occasions. The average hammer price has been 25 euros, suggesting that there has not been a significant increase in its value in the past.
4. Previous prices: In previous auctions, the starting price has varied, but the hammer price has consistently been low, especially for the current condition.

Given that the condition of the coin is low (30), and considering that the average hammer price has been 25 euros, it is reasonable to set a maximum purchase price that does not exceed the starting price, as demand and interest in coins in this condition tend to be limited.

Estimated maximum purchase price: 30 euros.

This price allows for a small margin over the starting price, but remains conservative given the sales history and the condition of the coin.