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

1. Current Condition: The coin has a condition rating of 20 (BC-), indicating that it is in quite low condition. This generally reduces its value compared to coins in better condition.

2. Current Listing Price and Previous Prices: The current listing price is 170 euros, and the average hammer price in previous auctions is 248.75 euros. However, previous hammer prices have varied significantly, with some reaching up to 1000 euros in more favorable conditions.

3. Price Trends: Looking at previous hammer prices, there is a trend where coins in similar conditions have reached hammer prices ranging from 120 to 440 euros, depending on the condition and demand at the time of the auction.

4. Analysis of Past Auctions: The coin has been auctioned 20 times and has a history of successful sales, suggesting that there is interest in this coin, although its current condition limits its appeal.

Given that the coin's condition is low (20), and considering the average hammer price of 248.75 euros, an estimated maximum purchase price could be set in a more conservative range.

Estimated Maximum Purchase Price: It may be reasonable to consider a maximum purchase price of around 150-180 euros, given that the condition significantly affects the value, but there is still interest in the coin. This allows for some margin for competition in the auction, without exceeding what could be considered excessive for a coin in this condition.