To estimate a maximum purchase price for the coin that will be auctioned, we will consider several factors, including the starting price, the condition of the coin, and the hammer prices of previous auctions for coins with similar conditions.

1. Starting price: 400 euros.
2. Condition of this coin: 90 (EF).
3. Previous hammer prices for coins with similar condition (90):
- In previous auctions, hammer prices for coins with a condition of 90 have varied, but they have generally sold between 300 and 400 euros, with some cases reaching up to 500 euros.

4. Analysis of previous prices:
- The average hammer price for coins with a condition of 90 in the past has been approximately 360 euros.
- The trend shows that coins with a condition of 90 can reach hammer prices above 400 euros in competitive auctions.

5. Estimation of the maximum purchase price:
- Given that the starting price is 400 euros and considering that the condition is good (90), a reasonable maximum purchase price could be in the range of 450 to 500 euros, depending on the demand at the auction.

Conclusion: An estimated maximum purchase price for this coin could be 450 to 500 euros.