To estimate a maximum purchase price for the coin in question, we can consider several factors based on the provided data:

1. Current Condition: The coin has a condition rating of 90 (EF), indicating that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Auction History:
- The average hammer price for this coin is 397.8 euros.
- In previous auctions, we have seen hammer prices that varied significantly, but many of them are above the current starting price of 300 euros.
- The most recent auction with a similar condition (90) had a hammer price of 650 euros.

3. Price Trends:
- The overall trend shows that coins with a condition of 90 have occasionally reached hammer prices exceeding 600 euros.
- The estimated price of 550 euros in a recent auction also suggests that the market is willing to pay more for coins in good condition.

4. Starting Price: The starting price is 300 euros, which can be considered a good starting point, but given the hammer price history, it is likely that the final price will be significantly higher.

Based on these factors, an estimated maximum purchase price for this coin could be between 600 and 700 euros. This is based on the hammer price history for coins of similar condition and market trends.