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

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 in previous auctions is 77.46 euros.
- There have been previous auctions where the coin has sold for prices that varied significantly, from 28.88 euros to 286.66 euros, depending on the condition and demand at that time.
- In particular, there is a hammer price of 286.66 euros for a coin with the same condition (90) in 2012.

3. Starting Price and Average Starting Price:
- The current starting price is 50 euros, which is relatively low compared to the average hammer price and the average starting price (79.32 euros).

4. Recent Trends: In the most recent auctions, hammer prices have been in a range of 50 to 60 euros for similar conditions, suggesting that interest may be fluctuating.

Based on these factors, an estimated maximum purchase price could be set in a range between 150 and 200 euros. This is based on the fact that, although the starting price is low, the condition of the coin and the price history suggest that it could reach a higher price in a competitive auction.

Therefore, a reasonable maximum purchase price could be 200 euros.