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

1. Current Condition: The coin has a condition rating of 80 (MS+). This is an important factor, as coins in better condition tend to reach higher prices.

2. Starting Price: The starting price is 400 euros. This can influence the final price, as buyers often consider the starting price as a reference point.

3. Sales History:
- The average hammer price in previous auctions is 283.35 euros.
- The average starting price in previous auctions is 241.78 euros.
- Previous auctions of coins with similar conditions (80-85) have had hammer prices that varied, but some have reached up to 350 euros in higher condition (95).

4. Recent Trends: In the most recent auctions, hammer prices have shown an increase, with some reaching up to 480 euros for coins in similar conditions.

5. Demand and Supply: The coin has been auctioned 270 times in the past, with a sell-through rate of 94.44% (255 out of 270). This indicates good demand, which can drive up the price.

Based on these factors, an estimated maximum purchase price could be between 400 and 450 euros, considering that the starting price is 400 euros and there is an upward trend in hammer prices in recent auctions. However, given that the condition is 80, it is unlikely to exceed 450 euros significantly unless there is intense competition among bidders.

Therefore, a reasonable maximum purchase price could be 450 euros.