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+), indicating that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Previous Prices: By looking at previous auction prices for coins with similar conditions (80 and 85), we can see that auction prices have varied. For example:
- In previous auctions with a condition of 80, the hammer prices have been around 290 to 550 euros.
- In auctions with a condition of 85, the hammer prices have reached up to 2900 euros, but this may be an exceptional case.

3. Starting Price and Average Hammer Price: The current starting price is 500 euros, and the average hammer price for this coin is 549.6 euros. This suggests that the market is willing to pay close to this price.

4. Market Trends: Since the coin has been auctioned 37 times and has had a sell-through rate of 97% (36 out of 37), this indicates strong demand.

5. Comparison with Historical Prices: In recent auctions, hammer prices for coins with similar conditions have been in the range of 500 to 850 euros.

Based on these factors, an estimated maximum purchase price for this coin could be between 600 and 700 euros, considering that the condition is good and there is a trend of increasing prices in recent auctions. However, given the starting price of 500 euros, it is likely that buyers will be willing to pay a bit more, but not excessively above this range.

Therefore, a reasonable maximum purchase price could be 700 euros.