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

1. Condition of the coin: The coin to be auctioned has a condition rating of 25 (BC), which indicates that it is in a fairly deteriorated state. This will negatively affect its value compared to coins in better condition.

2. Previous auction prices: By observing the hammer prices of similar coins in the past, we can see that prices have varied. For example:
- In 2019, a coin with a condition rating of 70 was sold for 17 euros.
- In 2022, a coin with a condition rating of 80 was sold for 90 euros.
- In 2023, a coin with a condition rating of 70 was sold for 75 euros.

3. Average hammer price: The average hammer price for previously auctioned coins is 47.1 euros, but this includes coins in better condition.

4. Starting price: The starting price of the coin is 20 euros, which may be a good indicator of its expected minimum value.

Given that the condition of the coin is quite low (25), and considering that coins in better condition have reached higher hammer prices, an estimated maximum purchase price could be set in a lower range.

Taking all these factors into account, an estimated maximum purchase price for this coin could be around 25-30 euros. This reflects a value that considers its condition and previous hammer prices, but also allows for a small margin above the starting price.