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

1. Current asking price: 1200 euros.
2. Current grade: 90 (XF), which is a good grade.
3. Auction history:
- In 2019, the coin with a grade of 85 was auctioned for a hammer price of 1711 euros.
- In 2021, the coin with a grade of 95 was auctioned for a hammer price of 3200 euros.

4. Average hammer price: 2455.5 euros, which is a good indicator of market value.

5. Average asking price: 1350 euros, which also suggests a reference value.

Given that the current grade of the coin is 90, which is higher than the 2019 auction (85) and slightly lower than the 2021 auction (95), we can infer that the hammer price should fall within an intermediate range.

Considering the hammer prices of 1711 euros (2019) and 3200 euros (2021), and the fact that the average hammer price is 2455.5 euros, we can estimate that a reasonable maximum purchase price for the coin in grade 90 could be between 2000 and 2500 euros.

Therefore, an estimated maximum purchase price could be approximately 2400 euros. This is based on the trend of previous hammer prices and the current grade of the coin.