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

1. Current Condition: The coin has a condition rating of 20 (BC-), indicating that it is in quite poor condition. This generally reduces the value of the coin compared to coins in better condition.

2. Auction History:
- The coin has been auctioned 12 times and has sold on all those occasions, indicating a consistent interest in the coin.
- The average hammer price in previous auctions is 62.73 euros, but this value includes coins in better condition.
- Previous hammer prices for coins with similar conditions (30 and 50) have ranged from 25 to 80 euros, suggesting that the price can vary significantly depending on the condition.

3. Starting Price and Estimates:
- The current starting price is 20 euros, which is relatively low.
- The average starting price in the past has been 31 euros, suggesting that the current starting price could be a good indicator of what is expected to be obtained.

Given that the condition of the coin is quite low (20), and considering the auction price history, an estimated maximum purchase price could be in the range of 25 to 40 euros. This is based on the fact that, although there is interest in the coin, its low condition will limit the price that collectors are willing to pay.

Therefore, an estimated maximum purchase price could be 35 euros.