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 an important factor, as coins in better condition tend to reach higher prices.

2. Auction History:
- The average hammer price for coins of similar condition (30) has been between 25 and 55 euros in previous auctions.
- In particular, there was a hammer price of 55 euros in December 2020 and other hammer prices around 30 euros in recent auctions.

3. Starting Price and Average Hammer Price:
- The current starting price is 20 euros, and the average hammer price in the past has been 38.9 euros.
- The average starting price has been 30.7 euros, suggesting there is room for the hammer price to be higher than the starting price.

4. Sales Trends: Out of the 36 times it has been auctioned, it has sold 24 times, indicating good demand.

5. Previous Estimates: In previous auctions, the estimated price has ranged from 30 to 100 euros, depending on condition and demand.

Based on these factors, an estimated maximum purchase price for this coin, considering its condition and auction history, could be around 50-60 euros. This is based on the highest hammer price observed for coins of similar condition and the general interest in the coin.