To estimate a maximum purchase price for the coin in question, let's consider the following factors:

1. Starting price: 100 euros.
2. Current condition: 90 (EBC), indicating good condition.
3. Auction history: The coin has been auctioned once before, with a hammer price of 120 euros and a starting price of 100 euros.
4. Average hammer price: 120 euros, which is the average price it has sold for in the past.
5. Auction expectations: It will be auctioned again in the coming days, which may influence demand.

Since the current condition is better than that of the previous auction (90 vs. 75), and considering that the average hammer price is 120 euros, we could estimate that the maximum purchase price could be in a range above the previous hammer price.

Taking all these factors into account, an estimated maximum purchase price could be approximately 130-150 euros. This reflects the good condition and potential interest in the coin, as well as the price history.