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

1. Condition of the coin: The coin to be auctioned has a condition of 90 (EBC), which indicates 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 for coins of this reference is 21.8 euros.
- In the past, coins with similar conditions have been auctioned, and the highest recorded hammer price was 95 euros (in 2014, with unspecified condition).
- The most recent auction with a condition of 95 reached a hammer price of 65 euros.

3. Starting price and previous prices:
- The current starting price is 30 euros, which is significantly higher than the average starting price of 12.94 euros. This may indicate an expectation of greater interest in this auction.
- The coin has been sold in the past at prices that varied, but the range seems to be between 5 and 95 euros, depending on the condition and market interest.

4. Market trends: Given that the coin has a condition of 90 and considering the interest in recent auctions, it is reasonable to assume that the hammer price could be above the historical average.

Based on these factors, an estimated maximum purchase price for this coin could be between 50 and 70 euros, considering that the condition is good and there is growing interest in the auction. However, given the starting price of 30 euros, it is possible that the final hammer price may not reach the estimated maximum, but it is a reasonable range to consider.