To estimate a maximum purchase price for the coin in question, we can analyze the provided data:

1. Starting price: 18 euros.
2. Coin condition: 30 (BC+), indicating a low condition.
3. Auction history: The coin has been auctioned 9 times, of which it has sold 8 times. This suggests there is a constant interest in the coin.
4. Average starting price: 29.22 euros.
5. Average hammer price: 40 euros.

### Analysis of previous prices:
- Previous hammer prices varied, but some approached 40 euros, which is the average hammer price.
- The current condition of the coin (30) is similar to the last auction in 2021, where the hammer price was 35 euros.

### Estimation:
Given that the condition is low and the starting price is 18 euros, but the average hammer price is 40 euros, we can consider that the hammer price could be in a range between 30 and 40 euros, depending on the interest of collectors in the auction.

Estimated maximum purchase price: It could be reasonable to set a maximum purchase price around 35-40 euros, given the sales history and interest in the coin. However, since the condition is only BC+, a more conservative price could be 30-35 euros.

Therefore, an estimated maximum purchase price could be 35 euros.