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

1. Current Condition: The coin has a condition of 30 (BC+), indicating that it is not in very good shape. This will affect its value compared to coins in better condition.

2. Previous Auction Prices:
- Previous hammer prices have varied significantly, ranging from 70 euros to 1800 euros, depending on the condition of the coin.
- Most coins with higher conditions (such as 90 and 100) have reached significantly higher hammer prices.

3. Starting Price and Average Price:
- The current starting price is 70 euros, and the average starting price in the past has been 176.25 euros, with an average hammer price of 420.75 euros. This suggests that, in general, coins of this reference tend to sell for much higher prices than their starting price.

4. Sales Trends: The coin has sold in 8 out of the last 9 auctions, indicating a consistent interest in this reference.

5. Comparison with Similar Coins: Coins with similar conditions (such as 20 and 30) have had hammer prices ranging between 70 and 220 euros, which can provide an idea of what to expect.

Given all this, an estimated maximum purchase price for this coin, considering its condition and previous hammer prices, could be in the range of 100 to 150 euros. This is based on the fact that, although the condition is not high, the interest in the coin and previous hammer prices suggest that it could reach a reasonable price at auction.