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

1. Starting price: 500 euros.
2. Coin condition: 90 (EBC), indicating good condition.
3. Auction history:
- It has been auctioned 4 times and sold on all occasions.
- The average hammer price in previous auctions is 508.75 euros.
- Previous hammer prices have been 480 euros (with condition 85) and 550 euros (with condition 80).

Given that the condition of the current coin is better (90) than the previous ones (85 and 80), it is reasonable to assume that the hammer price could be higher than the previous ones.

### Price Analysis:
- The average hammer price (508.75 euros) is a good indicator of the current value.
- The last auction with condition 80 reached 550 euros, suggesting that a coin in better condition could exceed this price.

### Estimation:
Taking all these factors into account, an estimated maximum purchase price could be between 550 and 600 euros, considering that the condition is better and that interest in the coin may have increased.

Therefore, a reasonable maximum purchase price could be 600 euros.