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

1. Current Condition: The coin has a condition of 70 (MS-). This is an important factor, as coins in better condition tend to reach higher prices.

2. Auction History:
- In the past, coins of the same reference with similar conditions (70 and 75) have been auctioned.
- Estimated prices for coins with a condition of 75 have reached up to 1300 euros, while for a condition of 70, estimated prices have been up to 500 euros.

3. Starting Price and Average Starting Price:
- The current starting price is 400 euros, which is lower than the average starting price of 500 euros. This may indicate a buying opportunity.

4. Sales Trends:
- Of the 5 times it has been auctioned in the past, it has only sold 2 times, which may indicate some resistance in the market for this coin.

5. Proximity of the Auction:
- There is an auction scheduled in the coming days, which may influence demand and price.

Based on these factors, an estimated maximum purchase price could be between 500 and 700 euros, considering that the condition is slightly lower than coins that have reached higher prices, but also taking into account the starting price and potential interest in the upcoming auction.

Therefore, a reasonable maximum purchase price could be 600 euros.