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. Previous Auction Prices: By observing the previous auction prices of coins with similar conditions, we can see that prices vary. For example, coins with conditions ranging from 25 to 30 have had auction prices ranging from 12 to 54 euros.

3. Average Auction Price: The average auction price for this coin is 116.27 euros, but this includes coins in better condition. Given that the condition of this coin is lower, it is likely that the auction price will be below this average.

4. Starting Prices and Estimates: The current starting price is 15 euros, and the average starting price in previous auctions is 80.21 euros. However, since the condition is low, it is unlikely to reach the average starting price.

5. Auction Trends: The coin has been auctioned 81 times, with an 81.5% sell-through rate. This indicates that there is considerable interest in the coin, but the auction price may be influenced by its condition.

### Estimated Maximum Purchase Price
Taking all these factors into account, an estimated maximum purchase price for this coin could be between 25 and 40 euros. This is based on the fact that, although the condition is low, there is interest in the coin and the starting price is relatively low.

Therefore, a reasonable maximum purchase price could be 35 euros.