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

1. Current starting price: 800 euros. This is the initial price set for the auction.
2. Condition of the coin: 75 (MBC). This indicates that the coin is in good condition, which may increase its value.
3. Historical data: In the previous auction, the coin sold for 266 euros with a starting price of 175 euros. This suggests that, although the starting price was low, the demand for the coin was sufficient to reach a reasonable hammer price.
4. Average starting and hammer price: The average starting price is 487.5 euros and the average hammer price is 266 euros. This indicates that, in general, coins of this reference tend to sell for less than their starting price.

Given that the coin is in good condition and has had a successful sale in the past, but also considering that the current starting price is significantly higher than the previous average hammer price, an estimated maximum purchase price could be set in a range between the average hammer price and a bit more, given the increase in condition.

Therefore, an estimated maximum purchase price could be around 350 to 400 euros. This reflects an interest in the coin, but also a caution given the difference between the starting price and the historical hammer prices.