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

1. Current Condition: The coin has a condition of 30 (BC+), which indicates that it is not in the best condition. This may negatively affect its value compared to coins in better condition.

2. Auction History:
- The average hammer price for similar coins in the past is 66.2 euros.
- Previous hammer prices for this same reference vary, but there was a hammer price of 30 euros in 2020 and another of 81 euros in 2020 for a better condition (80).
- The trend shows that coins in better condition have reached higher prices.

3. Starting Price and Estimates:
- The current starting price is 15 euros, which may attract more bidders.
- The estimated price of 25 euros in a previous auction with a similar condition is also relevant.

4. Demand and Supply: The coin has been auctioned 6 times in the past, of which 5 have sold. This indicates a consistent interest in the coin, which may increase its value.

Taking all these factors into account, a reasonable maximum purchase price could be in the range of 30 to 40 euros, considering that the condition is not optimal and that the highest hammer price in the past was 81 euros for a better condition.

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