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

1. Current starting price: 150 euros.
2. Auction history: The coin has been auctioned 2 times in the past, and on both occasions, it was sold. This indicates interest in the coin.
3. Average starting and hammer price: The average starting price is 300 euros and the average hammer price is 700 euros. This suggests that, in general, coins of this reference tend to reach higher prices at auctions.
4. Condition: There is no information about the current condition of the coin, but in the previous auction, a coin with a condition value of 75 sold for 750 euros. This indicates that if the condition is similar or better, the price could be high.
5. Price trends: Given that the previous hammer price was 750 euros, this sets a high benchmark.

Since the current starting price is significantly lower than the average hammer price and the previous hammer price, and considering that the coin has a good sales history, an estimated maximum purchase price could be positioned in a range between the average hammer price and the previous hammer price.

Estimated maximum purchase price: It could be reasonable to consider a maximum purchase price of around 700 to 750 euros, assuming that the condition is at least similar to that of the previous auction. However, since there is no information about the current condition, a more conservative approach could be to set a limit around 600 euros to avoid overpaying if the condition is not favorable.

Therefore, an estimated maximum purchase price could be 700 euros.