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

1. Starting price: 150 euros.
2. Auction history: The coin has been auctioned 15 times, of which it has sold 11 times. This indicates good demand.
3. Average hammer price: 122.91 euros, suggesting that, on average, coins of this reference sell for a price lower than the current starting price.
4. Condition: There is no specific information about the condition of this coin, which may make buyers more cautious.
5. Previous prices: Hammer prices in previous auctions varied, but there are examples of sales reaching up to 250 euros under favorable conditions.

Given that the starting price is 150 euros and considering the average hammer price of 122.91 euros, an estimated maximum purchase price could be set in a range that reflects both demand and uncertainty regarding the condition.

Taking all these factors into account, an estimated maximum purchase price could be approximately 180 euros. This price is higher than the average hammer price but lower than the starting price, reflecting a balance between historical demand and the lack of information about the current condition.