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

1. Starting price: 200 euros.
2. Auction history: The coin has been auctioned 23 times, of which it has sold 20 times. This indicates good demand.
3. Average hammer price: 115.58 euros, which is significantly lower than the current starting price.
4. Previous prices: Hammer prices have varied, with some reaching up to 301 euros in a recent auction, but there are also lower prices, such as 55 euros in other auctions.

Since there is no information about the condition of this coin, it is difficult to make an accurate estimate. However, we can consider the following points:

- The starting price of 200 euros is quite high compared to the average hammer price of 115.58 euros.
- The trend of hammer prices in the past shows that, although there are exceptions, most coins have sold for below the current starting price.

Based on these factors, an estimated maximum purchase price could be in a range between the average hammer price and the starting price. A reasonable approach would be to consider a maximum purchase price of around 150-180 euros, as this is above the average hammer price but still below the starting price, which could be attractive to a buyer looking for good value.

Therefore, the estimated maximum purchase price could be 180 euros.