To estimate a maximum purchase price for the coin in question, let's consider the following factors:

1. Starting price: 45 euros.
2. Average hammer price: 80 euros. This is an important figure, as it indicates the price at which it has sold in the past.
3. Coin condition: 20 (BC-). This indicates that the coin is in quite low condition, which generally negatively affects its value.
4. Auction history: The coin has been auctioned once and sold on that occasion, suggesting that there is interest in the coin, even though its condition is not high.

Given that the average hammer price is 80 euros, but the condition is low (20), it is likely that the hammer price in this auction will not reach that average.

Taking into account the condition and the starting price, an estimated maximum purchase price could be set between the starting price and the average hammer price, but adjusted downward due to the condition of the coin.

Therefore, a reasonable maximum purchase price could be around 60-70 euros, considering that the condition affects the value, but there is also demonstrated interest in the coin.