To estimate a maximum purchase price for the coin in question, we will consider several factors:

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

2. Auction History:
- The average hammer price for similar coins in the past has been 313.51 euros.
- In previous auctions, coins with a condition of 90 have reached hammer prices that varied, but some examples are:
- 341 euros (2020-10-21)
- 380 euros (2020-05-26)
- 360 euros (2021-06-22)
- 400 euros (2022-03-10)
- 440 euros (2023-03-16)

3. Starting Price and Estimated Price:
- The current starting price is 300 euros, and the estimated price is 500 euros. This suggests that there are expectations for the coin to sell at a considerably higher price.

4. Recent Trends: In the most recent auctions, hammer prices have been in a high range, indicating increasing demand.

Based on these factors, an estimated maximum purchase price could be in the range of 400 to 500 euros, considering that the condition is good and there is a history of hammer prices that supports this estimate.

Therefore, a reasonable maximum purchase price could be 450 euros.