To estimate a maximum purchase price for the coin in question, we can analyze the provided data, especially the previous auction prices and the current condition of the coin.

1. Current Condition: The coin has a condition of 70 (VF-). This indicates that it is in an acceptable state, but not of high quality.

2. Previous Auction Prices:
- The auction prices for coins with similar conditions (70) have varied, but we can observe that in previous auctions, the hammer prices have ranged between 10 and 110 euros, depending on demand and other factors.
- For example, in previous auctions with a condition of 70, the hammer prices have been 10, 20, 28, 90, and 110 euros.

3. Average Hammer Price: The average hammer price for this coin is 107.94 euros, suggesting that, in general, coins of this reference tend to reach higher prices.

4. Starting Price: The starting price is 15 euros, which is relatively low compared to previous hammer prices.

5. Auction Trends: Given that there have been 19 previous auctions and 18 sales, this indicates a consistent interest in this coin.

Based on these factors, an estimated maximum purchase price for this coin could be in the range of 80 to 120 euros, considering that the condition is 70 and the average hammer price is 107.94 euros.

Therefore, a reasonable estimated maximum purchase price could be 100 euros.