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-), indicating that it is in quite good shape. This can positively influence its value.

2. Auction History:
- In the 2021 auction, the coin was estimated at 9 euros and sold for 8 euros, suggesting limited interest at that time.
- In the 2023 auction, the coin had the same condition (70) and was estimated at 40 euros, selling for 30 euros. This indicates an increase in interest and value of the coin compared to the previous auction.

3. Starting Price and Average Prices:
- The current starting price is 18 euros, which is lower than the average hammer price of 19 euros in previous auctions. This suggests that the starting price is reasonable and could attract buyers.
- The average starting price in the past has been 21.5 euros, which also indicates potential for an increase in the hammer price.

4. Current Demand: The coin will be auctioned in the coming days, which may generate additional interest and potentially increase the hammer price.

Taking all these factors into account, an estimated maximum purchase price could be around 30-35 euros, based on the hammer price of the previous auction with the same condition and the growing interest in the coin. This would allow a buyer to be in a competitive position without exceeding the recent historical value.