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 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 has been 47 euros.
- In previous auctions, coins with a similar condition (90) have reached hammer prices of up to 50 euros.

3. Starting Price: The starting price is 20 euros, which is relatively low compared to previous hammer prices.

4. Price Trends: Observing previous hammer prices, it seems there is a trend for coins in good condition (EF) to sell for above 40 euros.

Based on these factors, an estimated maximum purchase price for this coin could be between 50 and 60 euros, considering that the condition is favorable and that the price history supports this estimate. Therefore, a reasonable maximum purchase price could be 55 euros.