To estimate a maximum purchase price for the coin in question, we can analyze the provided data:

1. Current starting price: 200 euros.
2. Coin condition: 90 (EBC), indicating good condition.
3. Auction history:
- In 2022, a coin with a condition of 80 was auctioned for a hammer price of 345 euros.
- In 2024, a coin with a condition of 95 was auctioned for a hammer price of 750 euros.

4. Average hammer price: 531.67 euros, suggesting that, in general, coins of this reference tend to reach higher prices at auction.

Since the condition of the coin to be auctioned is 90, which is higher than the 2022 auction (80) and lower than the 2024 auction (95), we can infer that the hammer price should be somewhere in between.

### Estimation of the maximum purchase price:
- Considering the hammer price of 345 euros for the condition of 80 and 750 euros for the condition of 95, we can perform an interpolation for the condition of 90.
- A simple approximation could be to calculate a weighted average between these two prices, taking into account that 90 is closer to 95 than to 80.

We can estimate the hammer price for the condition of 90 as:

\[
\text{Estimated price} = \frac{(345 + 750)}{2} = 547.5 \text{ euros}
\]

However, given that the average hammer price is 531.67 euros, and considering that the coin has a starting price of 200 euros, a reasonable maximum purchase price could be slightly higher than the average hammer price.

### Estimated maximum purchase price:
Estimated maximum purchase price: 550 euros.

This price reflects the good condition of the coin and the price history in previous auctions.