To estimate a maximum purchase price for the coin in question, we can consider several factors based on the provided data:

1. Condition: The coin has a condition of 70 (VF-), indicating that it is in relatively good shape. This can positively influence its value.

2. Previous auction prices:
- In December 2018, the coin was auctioned with a starting price of 300 euros, although the final price is not reported.
- In November 2018, the starting price was 150 euros, and it sold for 350 euros.
- In May 2023, the starting price was 100 euros, with an estimated price of 200 euros, but the final price is not reported.

3. Average starting and final prices: The average starting price is 152.5 euros, and the average final price is 350 euros. This suggests that, in general, coins of this reference tend to sell for a significantly higher price than their starting price.

4. Sales history: The coin has been auctioned 4 times, of which it has only sold once. This may indicate that there is limited interest in this specific coin, which could affect its price.

Given that the condition is good and considering the previous auction prices, an estimated maximum purchase price could be in a range between the average final price and the highest recorded starting price.

Taking all these factors into account, an estimated maximum purchase price could be around 300-350 euros. This is based on the fact that the highest recorded final price was 350 euros and the condition of the coin is favorable.