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 poor condition. This will negatively affect its value compared to coins in better condition.

2. Auction History: By observing the hammer prices of similar coins in better condition, we can see that prices have ranged between 75 and 250 euros, depending on the condition. Most coins auctioned in higher conditions (70-85) have reached hammer prices between 90 and 250 euros.

3. Starting Prices and Average Prices: The current starting price is 50 euros, and the average starting price in the past has been 100.5 euros, with an average hammer price of 160 euros. However, given that the condition of this coin is significantly lower, it is likely that the hammer price will be below the average.

4. Sales Trends: The coin has been auctioned 14 times, of which 12 have resulted in sales. This indicates consistent interest, but it also suggests that the price may be influenced by supply and demand in the market.

Taking all these factors into account, an estimated maximum purchase price for this coin, considering its low condition and price history, could be around 70-90 euros. This would allow a potential buyer to consider the possibility of future appreciation, but it also reflects the reality of its current state.