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 of 30 (BC+). This indicates that it is not in the best condition, which will affect its value.

2. Previous Auction Prices: By looking at previous auction prices for coins with similar conditions, we can see that:
- In 2019, a coin with a condition of 30 was auctioned for 11 euros.
- In 2020, a coin with a condition of 25 was auctioned for 17 euros.
- In 2020, another coin with a condition of 30 was auctioned for 17 euros.

3. Average Auction Price: The average auction price for this coin is 86.5 euros, but this includes coins in better conditions. Given that the condition of the coin in question is quite low, it is unlikely to reach that price.

4. Starting Price and Average Starting Price: The starting price is 15 euros, and the average starting price is 42.43 euros. This suggests that interest in the coin may be moderate.

5. Sales Trends: The coin has been auctioned 83 times, of which 79 have been sold. This indicates a good level of interest in the coin, although the auction price can vary significantly depending on the condition.

### Estimation of Maximum Purchase Price
Given that the condition is 30 (BC+), and considering the previous auction prices for coins with similar conditions, a reasonable maximum purchase price could be in the range of 15 to 25 euros.

However, since the average auction price for coins in similar conditions has been around 17 euros, an estimated maximum purchase price could be 20 euros. This allows for some margin for competition in the auction, but also reflects the condition of the coin.