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

1. Current Grade: The coin has a grade of 30 (BC+). This is relatively low compared to other coins that have been auctioned in the past, where some reached higher grades (for example, 70 or 80).

2. Previous Auction Prices: Looking at previous auction prices for coins with similar grades:
- In 2022, a coin with a grade of 30 was auctioned for 81 euros.
- In 2023, another coin with a grade of 80 was auctioned for 110 euros, indicating that coins in better condition can fetch higher prices.

3. Average Auction Price: The average auction price for this coin is 90.88 euros, suggesting that, in general, coins of this reference tend to sell for relatively high prices.

4. Auction Trends: Since this coin has been auctioned 16 times and all have sold, there is a consistent demand. This may influence the final auction price.

5. Starting Price: The starting price is 30 euros, which may attract more bidders, but the final price will depend on the competition in the auction.

Considering all these factors, an estimated maximum purchase price for this coin could be in the range of 70 to 90 euros. This is based on the average auction price and previous auction prices for coins with similar grades. However, since the grade is an important factor, it is likely that the price will be closer to the lower end of this range.

Therefore, a reasonable estimated maximum purchase price could be 80 euros.