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

1. Current Condition: The coin has a condition of 30 (BC+). This is an important factor, as coins in better condition tend to reach higher prices.

2. Historical Prices: By observing previous auction prices, we can see that the coin has had significant price variations depending on its condition. For example:
- In 2019, a coin with a condition of 90 was auctioned for 160 euros.
- In 2020, a coin with a condition of 70 was auctioned for 35 euros.
- In 2022, a coin with a condition of 70 was auctioned for 60 euros.

3. Average Auction Price: The average auction price for this coin is 65.73 euros, which gives us an idea of the general value it moves within.

4. Starting Price and Average Starting Price: The current starting price is 20 euros, and the average starting price is 42.38 euros. This suggests that the starting price is relatively low compared to the average auction price.

5. Sales Trends: The coin has been auctioned 16 times, of which it has sold 15 times, indicating good demand.

Based on these factors, and considering that the condition of 30 (BC+) is lower than the higher conditions that have reached significant auction prices, an estimated maximum purchase price could be in the range of 40 to 60 euros.

Therefore, a reasonable estimated maximum purchase price could be 50 euros. This is based on the analysis of previous auction prices and the current condition of the coin.