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

1. Current Condition: The coin has a condition of 30 (BC+), which indicates that it is not in the best condition. This generally reduces its value compared to coins in better condition.

2. Auction History:
- The average hammer price for similar coins in the past has been 73.25 euros.
- Previous auctions of coins with similar conditions (around 30) have had hammer prices that varied, but generally, hammer prices for higher conditions (like 70-80) have reached up to 110 euros.

3. Starting Price and Estimated Price:
- The current starting price is 50 euros, and the estimated price is 90 euros. This suggests that there are expectations for the coin to sell for a higher price than the starting price.

4. Sales Trends: The coin has been sold in 88% of past auctions, indicating good demand.

Based on these factors, a reasonable maximum purchase price could be in the range of 70 to 90 euros, considering that the condition is not optimal and the average hammer price is 73.25 euros.

Therefore, an estimated maximum purchase price could be 80 euros. This allows for a margin for competition in the auction, given that the starting price is 50 euros and the estimated price is 90 euros.