To estimate a maximum purchase price for the coin in question, we can analyze the provided data, especially the previous auction prices and the conditions of the auctioned coins.

1. Current starting price: 300 euros.
2. Previous auction prices:
- The highest auction was 1200 euros in 2018 with a condition of 100.
- Other relevant auction prices include 320 euros in 2023 with a condition of 95 and 260 euros in 2022 with a condition of 95.
3. Condition of the coin: No information is provided about the current condition, which makes the estimation more complicated. However, we can observe that coins with higher conditions tend to reach higher auction prices.

Given that the starting price is 300 euros and considering that significantly higher auction prices have been reached in the past, especially for coins in good condition, an estimated maximum purchase price could be in the range of 400 to 600 euros, depending on the demand and the condition of the coin in the current auction.

Therefore, an estimated maximum purchase price could be approximately 600 euros.