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

1. Coin Condition: The coin to be auctioned has a condition of 30 (BC+), which indicates that it is in a relatively low state compared to other coins that have been auctioned.

2. Previous Auction Prices: By observing the hammer prices of coins with similar conditions, we can see that prices vary considerably. For example, in previous auctions, coins with conditions of 30 have reached hammer prices of up to 80 euros, while coins in better condition have reached much higher prices.

3. Average Hammer Price: The average hammer price for this reference is 192.22 euros, but this includes coins in better conditions. Given that the condition of the coin in question is quite low, it is unlikely to reach that price.

4. Starting Price and Estimated Prices: The starting price is 70 euros, and the average starting price in previous auctions is 117.22 euros. This suggests that the starting price is reasonable, but given the condition, the hammer price could be lower.

5. Sales Trends: The coin has been sold 90% of the times it has been auctioned, indicating good demand.

Taking all these factors into account, an estimated maximum purchase price for this coin could be between 80 and 100 euros, considering that the condition is a limiting factor and that the starting price is 70 euros. This would allow a potential buyer to make a competitive offer without exceeding the expected market value for a coin in this state.