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

1. Current Condition: The coin has a condition of 70 (VF-). This is an important factor, as coins in better condition tend to reach higher prices.

2. Auction History:
- The coin has been auctioned 6 times, of which it has sold 4 times. This indicates an interest in the coin.
- Previous hammer prices for coins with a similar condition (70) have been 750 euros in 2022 and 750 euros in 2021.
- For coins with a condition of 80, the hammer prices have been 750 euros and 700 euros, suggesting that condition has a significant impact on price.

3. Starting Price and Average Hammer Price:
- The current starting price is 500 euros, and the average hammer price in the past has been 919 euros. This suggests that the hammer price could be above the starting price.

4. Price Trends:
- The estimated price for the next auction is 1500 euros, but this refers to a condition of 90, which is significantly better than the current condition of 70.

Based on these factors, an estimated maximum purchase price could be in the range of 750 to 900 euros, considering that the condition of 70 may limit the price, but the interest in the coin and the auction history suggest it could reach a higher price than the starting price.

Therefore, a reasonable maximum purchase price could be 850 euros.