Based on the provided data, we can conduct an analysis to estimate a maximum purchase price for the coin in question.

1. Starting price: 9000 euros.
2. Average hammer price in the past: 15000 euros.
3. Condition of the coin: 100 (Uncirculated), indicating that it is in excellent condition.
4. Auction history: It has been auctioned once before, with a hammer price of 15000 euros.

Given that the coin has a high condition rating and has reached a hammer price of 15000 euros in the past, it is reasonable to consider that interest in this coin could be high in the upcoming auction.

Estimation of the maximum purchase price:
- Since the average hammer price has been 15000 euros and considering that the coin is in the same condition as in the previous auction, a maximum purchase price could be around 15000 euros, as this is the price it has previously reached.

However, since the starting price is 9000 euros, it is possible that bidders may be willing to pay more, but the hammer price may not significantly exceed the previous price due to factors such as current demand and the number of bidders.

Therefore, an estimated maximum purchase price could be 15000 euros, but it is advisable to pay attention to the dynamics of the auction and the competition from other bidders.