To estimate a maximum purchase price for the coin in question, we can analyze the provided data, including the current condition of the coin, previous auction prices, and behavior in past auctions.

1. Current Condition: The coin has a condition rating of 70 (VF-). This indicates that it is in an acceptable state, but not of high quality.

2. Previous Auction Prices:
- Previous auction prices for coins with similar conditions (70-80) have varied. For example:
- In 2017, a coin with a condition of 70 was auctioned for 17 euros.
- In 2022, a coin with a condition of 70 was auctioned for 21 euros.
- In 2021, a coin with a condition of 75 was auctioned for 30 euros.
- The average auction price for this coin is 38.47 euros, but this includes coins in better condition.

3. Starting Price and Average Starting Price:
- The current starting price is 25 euros, and the average starting price in previous auctions is 21.22 euros.

4. Sales Trends:
- Out of the 23 times it has been auctioned, it has sold 20 times, indicating good demand.

5. Estimation:
- Given that the condition is 70 and considering previous auction prices, an estimated maximum purchase price could be between 25 and 35 euros. This is based on the fact that the starting price is 25 euros and that, in previous auctions, coins with similar conditions have reached auction prices of up to 30 euros.

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