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

1. Current Condition: The coin has a condition of 30 (BC+), which indicates that it is not in very good shape. This will negatively affect its value compared to coins in better condition.

2. Auction History: By looking at previous auction prices, we can see that coins in better condition (for example, 85 and 95) have reached hammer prices of up to 220 euros. However, coins with lower condition have had lower hammer prices.

3. Average Hammer Price: The average hammer price for this coin is 102.91 euros, which gives us an idea of the general market value.

4. Starting Price: The current starting price is 12 euros, suggesting that the seller has low expectations compared to the average hammer price.

5. Sales Trends: The coin has been auctioned 12 times, of which 11 have been sold, indicating good demand in the market.

Given that the condition of the coin is relatively low (30), and considering the auction price history, an estimated maximum purchase price could be set in a lower range than the average hammer price.

Taking all these factors into account, an estimated maximum purchase price could be around 40-50 euros. This reflects a balance between the current condition and the price history, as well as the demand observed in previous auctions.