To estimate a maximum purchase price for the coin in question, we can consider several factors based on the provided data:

1. Current Condition: The coin has a condition rating of 30 (BC+). This is relatively low compared to other coins that have been auctioned, which may affect its value.

2. Auction History:
- The average hammer price for coins with a similar condition (30) in the past has been approximately 20-30 euros.
- In previous auctions, hammer prices for coins with a condition of 30 have varied, but many have reached between 12 and 40 euros.

3. Starting Price and Average Starting Price:
- The current starting price is 12 euros, and the average starting price in the past has been 25.9 euros. This suggests that the starting price is relatively low compared to the historical average.

4. Sales Trends:
- Out of the 54 times it has been auctioned, it has sold 31 times, indicating a reasonable sell-through rate. This may influence demand and, therefore, the price.

5. Previous Prices:
- In past auctions, the estimated price for coins with a condition of 30 has been up to 20-30 euros, and some have reached up to 40 euros at auction.

Based on these factors, an estimated maximum purchase price for this coin could be between 20 and 30 euros, considering that the condition is a limiting factor. However, if there is particular interest in this coin or if the auction generates competition, the price could exceed 30 euros, but it is unlikely to reach 40 euros given its condition.

Therefore, a reasonable maximum purchase price could be 30 euros.