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

1. Condition: The coin has a condition rating of 95 (S/C-), indicating that it is in very good shape. This generally increases its value compared to coins with lower conditions.

2. Auction history:
- In the December 2021 auction, the coin was estimated at 1000 euros and sold for 600 euros, with a condition of 80.
- In the May 2023 auction, the coin had a condition of 95, with a starting price of 3000 euros and an estimated price of 5000 euros, although it did not sell.

3. Current starting price: The starting price is 1500 euros, which is significantly lower than the starting price of the previous auction (3000 euros) and the average starting price (2250 euros).

4. Average hammer price: The average hammer price of 600 euros from the previous auction suggests that, although the coin has a high estimated value, market interest may not be as strong.

5. Market trends: Since the coin has only been auctioned twice and has had a low hammer price compared to its estimated price, collector interest may not be as high as expected.

Taking all these factors into account, an estimated maximum purchase price could be positioned between the current starting price and the average hammer price of the previous auction.

Therefore, a reasonable maximum purchase price could be around 1200 to 1500 euros, considering that the condition is high and the starting price is 1500 euros. This allows for some competition in the auction, but also reflects the reality of market interest.