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

1. Condition: The coin has a condition of 80 (MS+), which indicates that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Previous Prices:
- In the 2020 auction, the coin sold for 360 euros with a condition of 80.
- In 2021, it was estimated at a price of 500 euros, but it sold for 460 euros, also with a condition of 80.
- In 2014, although the condition is not specified, the final price was 340 euros.

3. Average Hammer Price: The average hammer price for this coin is 390 euros, which can serve as a good indicator of market value.

4. Current Starting Price: The starting price is 250 euros, which is reasonable and could attract more bidders.

Given that the condition is good and considering the previous hammer prices, an estimated maximum purchase price could be around 400-460 euros, which is the range where they have sold in the past.

Therefore, a reasonable maximum purchase price could be 450 euros. This allows for a margin to compete in the auction, considering that the final price may vary depending on demand at the time of the auction.